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Firefox for Web Developers

@webdevs.firefox.com
3.1K followers 8 following 470 posts

Official Firefox account for people who build on the web. Learn about the things we're working on to grow and improve the web platform.

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Firefox for Web Developers @webdevs.firefox.com · 29/09/2026
Firefox 157 has landed, and includes a couple of useful CSS features: 🎉 at-rule() in @supports - lets you limit rules to browsers that support a given at-rule. 🎉 overscroll-behavior: chain - lets scrolling pass up to parent elements, but prevents UA effects like 'bounce' and overscroll. …And more!
developer.mozilla.org
Firefox 157 release notes for developers (Stable) - Mozilla | MDN
This article provides information about the changes in Firefox 157 that affect developers. Firefox 157 was released on September 29, 2026.
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Firefox for Web Developers @webdevs.firefox.com · 10/09/2026
When and how CSS custom property values are calculated might not be what you expect, and it has a big impact on your styles…
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Firefox for Web Developers @webdevs.firefox.com · 10/09/2026
When and how CSS custom property values are calculated might not be what you expect, and it has a big impact on your styles…
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Reposted by Firefox for Web Developers
Jakob E @jakob-e.bsky.social · 08/09/2026
Great news 🤩 For anyone interested in how to use progress in fluid typography @matthewmorete.com made an excellent post about it here 👇 blog.master.dev/fluid-typogr...
blog.master.dev
Fluid Typography with progress()
With the `progress()` function in CSS we've got a new way to calculate the size for type based on the viewport without problems of the past.
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Firefox for Web Developers @webdevs.firefox.com · 08/09/2026
The CSS progress() function is now in all major engines! Here's how it works:
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Firefox for Web Developers @webdevs.firefox.com · 03/09/2026
Interop 2027 is open for proposals! Is there a well-tested feature that you'd like to see cross-browser focus on next year? Submit it here: github.com/web-platform...
github.com
interop/proposal_guide.md at main · web-platform-tests/interop
web-platform-tests Interop project. Contribute to web-platform-tests/interop development by creating an account on GitHub.
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Firefox for Web Developers @webdevs.firefox.com · 03/09/2026
CSS attr() now works on all properties in Firefox 155. Here's how it works, and how to get the most out of it:
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rem / Remy Sharp @remysharp.com · 02/09/2026
I love this tool @webdevs.firefox.com devtools - so useful in the browser as a drag and design tool. Also, new speaker drop… "drop", announcement I think…
A screenshot of Firefox DevTools with the Polygon shape tool enabled updating the CSS in real time.
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Firefox for Web Developers @webdevs.firefox.com · 01/09/2026
Major versions of Firefox now land every two weeks, so Firefox 155 dropped today with web platform features including: 🎉 Fully typed CSS attr() 🎉 CSS progress() 🎉 Promise.allKeyed() and allSettledKeyed() 🎉 Lots of new WebTransport stuff …And more! developer.mozilla.org/en-US/docs/M...
developer.mozilla.org
Firefox 155 release notes for developers (Stable) - Mozilla | MDN
This article provides information about the changes in Firefox 155 that affect developers. Firefox 155 was released on September 1, 2026.
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Firefox for Web Developers @webdevs.firefox.com · 25/08/2026
CSS sibling-count() and sibling-index() are newly available across engines! Here's how they work:
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Firefox for Web Developers @webdevs.firefox.com · 24/08/2026
JPEG XL is slated to ship in Firefox 157 next month! Here's how we're shipping it safely… hacks.mozilla.org/2026/08/inte...
hacks.mozilla.org
Intent to Ship: JPEG XL – Mozilla Hacks - the Web developer blog
A JPEG XL decoder is heading for Firefox 157! Here's how we're shipping it safely…
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Firefox for Web Developers @webdevs.firefox.com · 24/08/2026
Intent to ship: JPEG XL! This will be on-by-default in Firefox 157, which should become the main release version at the end of September. This includes key JPEG XL features like progressive rendering. groups.google.com/a/mozilla.or...
groups.google.com
Intent to ship: JPEG XL
As of Firefox 157 I intend to turn JPEG XL decoding on by default on all platforms. It has been developed behind image.jxl.enabled, which today is on by default on Nightly only, and has had a Firefox…
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Firefox for Web Developers @webdevs.firefox.com · 20/08/2026
What should happen with text-box-trim and margin-trim when it comes to negative values? Do negatives 'trim'?
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Firefox for Web Developers @webdevs.firefox.com · 19/08/2026
CSS text-box is newly baseline, and it's solves a common text layout issue. Here's how it works:
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Firefox for Web Developers @webdevs.firefox.com · 18/08/2026
Firefox 154 landed today, and brings with it some great web platform features: 🎉 CSS text-box 🎉 CSS sibling-index() and sibling-count() 🎉 JS Iterator methods like chunks, window, includes, & join 🎉 HTTP No-Vary-Search …And more! developer.mozilla.org/en-US/docs/M...
developer.mozilla.org
Firefox 154 release notes for developers (Stable) - Mozilla | MDN
This article provides information about the changes in Firefox 154 that affect developers. Firefox 154 was released on August 18, 2026.
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Firefox for Web Developers @webdevs.firefox.com · 11/08/2026
We have an open position for a student worker on the Firefox Networking team in Germany. We work on the full stack, from high level HTTP all the way down to TCP/UDP. Some C++, some Rust. If that interests you, here are the details: www.mozilla.org/en-US/career...
mozilla.org
Mozilla Careers — Necko Student Worker — Open Positions
Mozilla is hiring a Necko Student Worker in Remote Canada, Firefox, New Products, Firefox, Core Services, Infrastructure, Mozilla.org, New Products, Marketing,…
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Firefox for Web Developers @webdevs.firefox.com · 04/08/2026
JS promise integration for WebAssembly landed in Firefox 153, making it easier to work with asynchronous calls from Wasm. Here's how it works:
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Firefox for Web Developers @webdevs.firefox.com · 30/07/2026
JS text imports landed in Firefox 153, making it the first browser to support this feature. Here's how it works:
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Jake Archibald @jakearchibald.com · 28/07/2026
I made an accessibility error when coding up some tooltips. Here's where I went wrong, and how you can avoid making the same mistake: jakearchibald.com/2026/my-tool...
jakearchibald.com
Fixing my tooltip accessibility mistake
aria-describedby isn't always enough.
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Firefox for Web Developers @webdevs.firefox.com · 27/07/2026
I made an accessibility error in one of my recent videos, and I want you to learn from my mistake: (sorry, yet another YT link - bsky video uploads are still broken) www.youtube.com/shorts/hPrZi...
youtube.com
Fixing my accessibility mistake
YouTube video by Firefox for Web Developers
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Firefox for Web Developers @webdevs.firefox.com · 23/07/2026
A little dive into web standards… Why changes were made to popover="hint", and why diversity in browser engines is important: (sorry again for the YT link - video uploads are still broken on bsky) youtube.com/shorts/9LDmd...
youtube.com
popover="hint" & engine diversity
YouTube video by Firefox for Web Developers
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Firefox for Web Developers @webdevs.firefox.com · 22/07/2026
popover="hint" landed in Firefox 153. Here's how it differs from regular popovers: youtube.com/shorts/HWetx... (sorry for the YouTube link, it seems like video uploads are broken on bsky today)
youtube.com
popover="hint" - new in Firefox 153
YouTube video by Firefox for Web Developers
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Firefox for Web Developers @webdevs.firefox.com · 22/07/2026
JS declarations in conditionals is being discussed at TC39 again. The tricky detail is, how should the "else" block be handled? I think Rust has the right answer here: bsky.app/profile/webd...
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Firefox for Web Developers @webdevs.firefox.com · 21/07/2026
Firefox 153 just landed, and comes with web platform features like: 🎉 popover="hint" 🎉 Picture-in-picture 🎉 IDB getAllRecords() 🎉 JS Promise Integration for wasm 🎉 JS import-as-text …And more! developer.mozilla.org/en-US/docs/M...
developer.mozilla.org
Firefox 153 release notes for developers (Stable) - Mozilla | MDN
This article provides information about the changes in Firefox 153 that affect developers. Firefox 153 was released on July 21, 2026.
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Firefox for Web Developers @webdevs.firefox.com · 16/07/2026
Someone said our Instagram page looks like "the world's most boring game of Guess Who" 😭 www.instagram.com/firefoxwebde...
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Firefox for Web Developers @webdevs.firefox.com · 15/07/2026
Ohhhh this has just been added to WebKit bugs.webkit.org/show_bug.cgi...
bugs.webkit.org
319178 – [css-text-decor-4] Add support for text-decoration-inset
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Firefox for Web Developers @webdevs.firefox.com · 09/07/2026
Notification actions landed in Firefox 152, allowing you to add quick action buttons to your notifications. Here's how they work:
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Firefox for Web Developers @webdevs.firefox.com · 17/06/2026
CSS field-sizing is newly baseline, meaning you can make various form fields size to their contents, without needing JavaScript.
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Firefox for Web Developers @webdevs.firefox.com · 16/06/2026
Firefox 152 landed today, and includes web platform features like: 🎉 CSS field-sizing 🎉 Notification actions 🎉 unadjustedMovement pointer locks 🎉 getAnimations() pseudoElement option …and more! developer.mozilla.org/en-US/docs/M...
developer.mozilla.org
Firefox 152 release notes for developers (Beta) - Mozilla | MDN
This article provides information about the changes in Firefox 152 that affect developers. Firefox 152 is the current Beta version of Firefox and ships on June 16, 2026.
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Firefox for Web Developers @webdevs.firefox.com · 15/06/2026
If your content is served over HTTP/3, make sure your DNS record advertises it, otherwise you're missing out on free performance. Check your site here: savearoundtrip.com
savearoundtrip.com
savearoundtrip
Publish an HTTPS DNS record. Let browsers reach HTTP/3 on the first connection instead of wasting a round trip.
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Firefox for Web Developers @webdevs.firefox.com · 02/06/2026
Ever struggled to get heading tags right in includes/components? headingoffset could be the answer…
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Firefox for Web Developers @webdevs.firefox.com · 01/06/2026
We're experimenting with a way to visualize session history in Firefox DevTools, and we'd love your feedback! Here's the background, and how to try it: blog.farre.se/posts/2026/0...
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Firefox for Web Developers @webdevs.firefox.com · 28/05/2026
The Web Serial API landed in Firefox 151, allowing you to connect directly to microcontrollers, dev boards, 3D printers, power meters, and other serial-connected hardware from the web. Here's how it works:
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Firefox for Web Developers @webdevs.firefox.com · 26/05/2026
Which of the following three CSS features would you prefer Firefox to support first? ❤️ your favorite in the thread:
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Firefox for Web Developers @webdevs.firefox.com · 21/05/2026
Web Serial landed in Firefox 151! Here's how it works, and what you can do with it: hacks.mozilla.org/2026/05/web-...
hacks.mozilla.org
Announcing Web Serial Support in Firefox – Mozilla Hacks - the Web developer blog
Support for Web Serial in Firefox 151 for Desktop Firefox can now connect directly to microcontrollers, development boards, 3D printers, power meters, and other serial-connected hardware from the…
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Firefox for Web Developers @webdevs.firefox.com · 20/05/2026
CSS style() queries landed in Firefox 151 making them newly available across engines. Here's how they work:
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Firefox for Web Developers @webdevs.firefox.com · 19/05/2026
Firefox 151 is out now, and brings with it some tasty new web platform features: 🎉 Container style() queries 🎉 Web Serial API 🎉 Document Picture-in-Picture API 🎉 keyboardLock on requestFullscreen …and more! developer.mozilla.org/en-US/docs/M...
developer.mozilla.org
Firefox 151 release notes for developers (Stable) - Mozilla | MDN
This article provides information about the changes in Firefox 151 that affect developers. Firefox 151 was released on May 19, 2026.
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Firefox for Web Developers @webdevs.firefox.com · 18/05/2026
Chrome shipped an LLM Prompt API to the web platform. At Mozilla, we oppose this API. Here's why:
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Firefox for Web Developers @webdevs.firefox.com · 07/05/2026
Recently, we fixed an unprecedented number of latent security bugs in Firefox identified by Claude Mythos Preview. Here's how we approached this work, what we found, and what we learned: hacks.mozilla.org/2026/05/behi...
hacks.mozilla.org
Behind the Scenes Hardening Firefox with Claude Mythos Preview – Mozilla Hacks - the Web developer blog
New details about what we found, and how agentic harnesses are now able to reproduce real bugs and dismiss false positives.
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Firefox for Web Developers @webdevs.firefox.com · 30/04/2026
Chrome looks set to ship an LLM Prompt API to the web platform. At Mozilla, we oppose this API. We feel it has a large interoperability risk, and Google imposing T&Cs on a web API sets a dangerous precedent. Full details: github.com/mozilla/stan...
github.com
Prompt API · Issue #1213 · mozilla/standards-positions
Specification title Prompt API Specification or proposal URL (if available) No response Explainer URL (if available) https://github.com/webmachinelearning/prompt-api/blob/main/README.md Proposal au...
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Maikel Krause @maikelkrause.bsky.social · 27/04/2026
One thing the example with the `@media` query doesn't quite capture (and I know this is meant to be a quick 1 min video), is that `light-dark()` will respect a "local" `color-scheme` override as well. So it's more than just a shorthand for the media query!
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Firefox for Web Developers @webdevs.firefox.com · 27/04/2026
CSS's light-dark() now accepts images, making it simpler to define alternate values for a dark color scheme. Here's how it works:
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Firefox for Web Developers @webdevs.firefox.com · 23/04/2026
sizes="auto" landed in Firefox 150, making responsive images simpler and more reliable. Here's when you can use it, and how…
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Firefox for Web Developers @webdevs.firefox.com · 21/04/2026
Firefox 150 just landed & includes new web platform features like: 🎉 sizes="auto" on images 🎉 light-dark() image support 🎉 CSS revert-rule 🎉 ariaNotify() 🎉 Media pseudo-classes like :playing None of these are baseline yet, so check MDN for support. developer.mozilla.org/en-US/docs/M...
developer.mozilla.org
Firefox 150 release notes for developers (Stable) - Mozilla | MDN
This article provides information about the changes in Firefox 150 that affect developers. Firefox 150 was released on April 21, 2026.
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Firefox for Web Developers @webdevs.firefox.com · 16/04/2026
map.getOrInsertComputed is newly baseline! It's a small feature, but I've found it pretty useful. Here's how it works:
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Firefox for Web Developers @webdevs.firefox.com · 15/04/2026
A common mistake with a 'subtree' MutationObserver is assuming addedNodes includes all newly connected nodes, but it doesn't include preexisting children of the added nodes codepen.io/editor/jaffa...
codepen.io
Mutation observer gotcha
A code demo by Jake Archibald created on CodePen
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Firefox for Web Developers @webdevs.firefox.com · 14/04/2026
CSS contrast-color is newly baseline, but it falls short in some areas. Here's how it works, and what to look out for:
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Firefox for Web Developers @webdevs.firefox.com · 14/04/2026
CSS contrast-color is newly baseline, but it falls short in some areas. Here's how it works, and what to look out for:
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Firefox for Web Developers @webdevs.firefox.com · 01/04/2026
I tried this. It's hard.
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Firefox for Web Developers @webdevs.firefox.com · 26/03/2026
hasUAVisualTransition landed in Firefox 149, making it available across all major engines. As well as having a very catchy name, it lets you avoid doubling up on page transitions. Here's how it works:
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