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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

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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Reposted by Firefox for Web Developers

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

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 · 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 · 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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Reposted by Firefox for Web Developers

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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