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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 · 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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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 · 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

Do you have some kind of customisation going on? Here's how it looks for me:

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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 · 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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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 · 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 · 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 · 18/05/2026

Correction: Chrome's doesn't use the cloud, it only uses the 4gb model. chrome://on-device-internals/ has a faulty "uninstall" feature, which made it look like the model was uninstalled, but it still worked, which made me think it (as the spec allows) was using the cloud.

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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 · 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 · 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 · 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 · 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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Firefox for Web Developers @webdevs.firefox.com · 25/03/2026

The CloseWatcher API landed in Firefox 149, making it easy to listen for platform-specific 'dismiss' signals. Here's how it works:

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Firefox for Web Developers @webdevs.firefox.com · 05/03/2026

JavaScript Iterator․zip landed in Firefox 148, making it simple to loop over multiple things at the same time. Here's how it works:

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Firefox for Web Developers @webdevs.firefox.com · 03/03/2026

If you have blocked AI enhancements, this is the same UI - the entry point to the feature isn't there. And of course, any already-downloaded models are deleted.

The UI as before, but with no button to suggest other tabs. The entry point has been removed.

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Firefox for Web Developers @webdevs.firefox.com · 03/03/2026

Taking, for example, the tab grouping and naming feature, if you don't "Block AI enhancements", then you're offered the feature, and if you click the button, the use of AI is explained, and you have to confirm. If you confirm, then the local privacy-preserving model is downloaded and used.

"Create tab group" UI, including a "Suggest more of my tabs" button featuring an icon typically indicating AI.

The UI when you click the button. Showing "Nightly uses AI to read your open tabs' titles and descriptions to suggest more tabs and group names. This happens on your device".

There are "cancel" and "continue" buttons, along with a link to customise AI settings.

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Firefox for Web Developers @webdevs.firefox.com · 02/03/2026

CSS shape() landed in Firefox 148. It's similar to path(), but allows the full expressiveness of CSS…

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Firefox for Web Developers @webdevs.firefox.com · 27/02/2026

If you have blocked AI enhancements, this is the same UI - the entry point to the feature isn't there. And of course, any already-downloaded models are deleted.

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Firefox for Web Developers @webdevs.firefox.com · 27/02/2026

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Firefox for Web Developers @webdevs.firefox.com · 25/02/2026

You can throw away innerHTML and replace it with the new setHTML(), which has a built-in sanitizer. Here's how it works:

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Firefox for Web Developers @webdevs.firefox.com · 17/02/2026

Here's the flow when you choose "suggest more of my tabs" when creating a tab group. AI models are not downloaded until you press "continue" in the second step.

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Firefox for Web Developers @webdevs.firefox.com · 16/02/2026

HTML might be getting a new type of tag, which… hasn't happened this millennium. Here's the new syntax, and how it works:

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Firefox for Web Developers @webdevs.firefox.com · 12/02/2026

Here's a quick rundown of what we delivered as part of Interop 2025, and what's planned for Interop 2026!

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Firefox for Web Developers @webdevs.firefox.com · 02/02/2026

AI Controls (formally 'kill switch') are landing in today's Firefox Nightly, and will land with Firefox 148 later this month.

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Firefox for Web Developers @webdevs.firefox.com · 02/02/2026

That's, of course, your choice. However, if you're interested, the latest nightly has the AI Controls (as in, the 'kill switch') behind the `browser.preferences.aiControls` `about:config` pref. It will be unflagged soon, and bumped to the beta version.

The current version of AI controls, in Firefox Nightly, where you can "Block AI enhancements".

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Firefox for Web Developers @webdevs.firefox.com · 23/01/2026

CSS module scripts landed in Firefox 147, so you can import CSS directly into JavaScript.

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Firefox for Web Developers @webdevs.firefox.com · 21/01/2026

In Firefox 147, you no longer need heavy libraries to (de)compress brotli with JS - you can just use CompressionStream and DecompressionStream!

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Firefox for Web Developers @webdevs.firefox.com · 19/01/2026

View Transition Types are newly baseline! These make it easier to build transitions that are specific to particular situations. Also, we have some new View Transition DevTools in Firefox 147…

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Firefox for Web Developers @webdevs.firefox.com · 15/01/2026

The Navigation API is newly baseline! The web now has sensible, low-level routing for navigations.

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Firefox for Web Developers @webdevs.firefox.com · 13/01/2026

CSS anchor positioning is newly baseline! This opens the door for lots of new creative CSS. It's a huge feature, but here are the basics.

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Firefox for Web Developers @webdevs.firefox.com · 19/12/2025

HTML hidden=until-found and auto-opening <details> is newly baseline… or is it? Here's how it works, and issues to watch out for:

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Firefox for Web Developers @webdevs.firefox.com · 17/12/2025

HTML invoker commands are newly baseline! Here's how they work…

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Firefox for Web Developers @webdevs.firefox.com · 15/12/2025

JavaScript declarations in conditionals was discussed at TC39 recently, but consensus broke down over one key detail. How do you think it should behave?

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Firefox for Web Developers @webdevs.firefox.com · 12/12/2025

In Firefox 146 DevTools, long lists of unused CSS custom properties collapse, decluttering the UI, and improving performance

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Firefox for Web Developers @webdevs.firefox.com · 11/12/2025

CSS text-decoration-inset landed in Firefox 146! Here's how it works:

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Firefox for Web Developers @webdevs.firefox.com · 09/12/2025

CSS @scope has landed in Firefox 146, making it a new baseline feature. Here's how it works:

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Firefox for Web Developers @webdevs.firefox.com · 05/12/2025

AV1 won an Emmy! We're proud to be part of the Alliance for Open Media, where the codec was developed. But the story's not over… AV2 is just around the corner. blog.mozilla.org/en/mozilla/av1-vid…

The Emmy award trophy, next to a cuddly toy Firefox fox

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Firefox for Web Developers @webdevs.firefox.com · 02/12/2025

Another API proposal from TPAC: CSS routes and navigation queries, meaning less JavaScript is needed for view transitions! drafts.csswg.org/css-navigation-1 github.com/w3c/csswg-drafts/issues/…

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