Firefox for Web Developers

developer.mozilla.orgFirefox 159 release notes for developers (Stable) - Mozilla | MDNThis article provides information about the changes in Firefox 159 that affect developers. Firefox 159 was released on October 27, 2026.

15h

Firefox 159 has landed! This is a pretty big one for web dev stuff: 🎉 CSS scroll-driven animations, both scroll() and view(). 🎉 Scoped custom element registries. 🎉 Streaming sanitization of HTML. And more! developer.mozilla.org/en-US/docs/M...

JPEG XL is newly available in all major engines, but how does it stack up against other formats?

What should we ship in 2027? Rank your favourite features & create your wishlist: interop-rank.fxdx.dev

developer.mozilla.orgFirefox 158 release notes for developers (Stable) - Mozilla | MDNThis article provides information about the changes in Firefox 158 that affect developers. Firefox 158 was released on October 13, 2026.

Firefox 158 landed today! For web devs: 🎉 JPEG XL support, including full progressive rendering. 🎉 'navigate' on notifications, so they can go to a URL without firing events. 🎉 WebGPU's float32-blendable 🎉 WebGL's EXT_polygon_offset_clamp And more! developer.mozilla.org/en-US/docs/M...

interop-rank.fxdx.dev2027 Web Platform Feature RankingRank the Interop 2027 features you care most about

We've loved seeing folks post their rankings, so we've added a share button! You can either share your list via the site directly, or copy your list as markdown/html for your own blog. interop-rank.fxdx.dev

interop-rank.fxdx.devInterop Feature RankingRank the web platform features you care most about

What do you want us to work on in 2027? Here's a web app where you can rank the Interop proposals you care about, giving us data we can use to prioritise feature development. interop-rank.fxdx.dev

We ran this last year to decide what to champion for Interop 2026, but the data was so good, we also used it to prioritise features beyond the Interop project. We've also added some features to make it quicker to rank items.

developer.mozilla.orgFirefox 157 release notes for developers (Stable) - Mozilla | MDNThis article provides information about the changes in Firefox 157 that affect developers. Firefox 157 was released on September 29, 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!

Firefox for Web Developers reposted

10 Sept

When and how CSS custom property values are calculated might not be what you expect, and it has a big impact on your styles…

blog.master.devFluid 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.

10 Sept

When and how CSS custom property values are calculated might not be what you expect, and it has a big impact on your styles…

Firefox for Web Developers reposted

Jakob E

@jakob-e.bsky.social

8 Sept

Replying to @webdevs.firefox.com

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...

8 Sept

The CSS progress() function is now in all major engines! Here's how it works:

developer.mozilla.orgprogress() CSS function - CSS | MDNThe progress() CSS function returns a <number> value representing the position of one value (the progress value) relative to two other values (the progress start and end values).

8 Sept

Full docs on MDN: developer.mozilla.org/en-US/docs/W...

github.cominterop/proposal_guide.md at main · web-platform-tests/interopweb-platform-tests Interop project. Contribute to web-platform-tests/interop development by creating an account on GitHub.

3 Sept

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...

3 Sept

CSS attr() now works on all properties in Firefox 155. Here's how it works, and how to get the most out of it:

developer.mozilla.orgattr() CSS function - CSS | MDNThe attr() CSS function is used to retrieve the value of an attribute of the selected element and use it in a property value, similar to how the var() function substitutes a custom property value. It…

3 Sept

The full docs on MDN: developer.mozilla.org/en-US/docs/W...

Firefox for Web Developers reposted

rem / Remy Sharp

@remysharp.com

2 Sept

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…

developer.mozilla.orgFirefox 155 release notes for developers (Stable) - Mozilla | MDNThis article provides information about the changes in Firefox 155 that affect developers. Firefox 155 was released on September 1, 2026.

1 Sept

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...

25 Aug

CSS sibling-count() and sibling-index() are newly available across engines! Here's how they work:

developer.mozilla.orgsibling-index() CSS function - CSS | MDNThe sibling-index() CSS function returns an integer representing the position of the current element in the DOM tree relative to all its sibling elements. The returned value is the index number of…

25 Aug

And the full docs on MDN: developer.mozilla.org/en-US/docs/W...

26 Aug

This was another feature added by @sajidanwar.com 🎉🎉🎉

hacks.mozilla.orgIntent to Ship: JPEG XL – Mozilla Hacks - the Web developer blogA JPEG XL decoder is heading for Firefox 157! Here's how we're shipping it safely…

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...

groups.google.comIntent to ship: JPEG XLAs 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…

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...

random-stuff.jakearchibald.comPartial image decoder

In a JPEG XL compatible browser (like Firefox Nightly), test how it renders progressively by moving the slider in this demo: random-stuff.jakearchibald.com/apps/partial...

groups.google.comIntent to Ship: JPEG XL decoding support (image/jxl) in blink

Chrome also has an Intent to Ship, so support should land in both browsers within a week of each other groups.google.com/a/chromium.o...

20 Aug

What should happen with text-box-trim and margin-trim when it comes to negative values? Do negatives 'trim'?

github.com[css-inline-3] Does `text-box-trim` remove negative leading? · Issue #14328 · w3c/csswg-draftsWhen the line-height is smaller than the font's ascent/descent, line boxes can have negative leading. If text-box-trim is applied to a block using a taller edge metric, does it remove this negative...

20 Aug

CSS issue: github.com/w3c/csswg-dr...

19 Aug

CSS text-box is newly baseline, and it's solves a common text layout issue. Here's how it works:

developer.mozilla.orgtext-box CSS property - CSS | MDNThe text-box CSS property is a shorthand that corresponds to the text-box-trim and text-box-edge properties, which together specify the amount of space to trim from the block-start edge and block-end ...

19 Aug

Full docs on MDN: developer.mozilla.org/en-US/docs/W...

19 Aug

The Firefox implementation of text-box was contributed by a community member, @sajidanwar.com. This is a feature I've been wanting for a long time, as I know many of you have too. So a huge thanks to Sajid for making it happen!

developer.mozilla.orgFirefox 154 release notes for developers (Stable) - Mozilla | MDNThis article provides information about the changes in Firefox 154 that affect developers. Firefox 154 was released on August 18, 2026.

18 Aug

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...

mozilla.orgMozilla Careers — Necko Student Worker — Open PositionsMozilla is hiring a Necko Student Worker in Remote Canada, Firefox, New Products, Firefox, Core Services, Infrastructure, Mozilla.org, New Products, Marketing,…

11 Aug

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...

4 Aug

JS promise integration for WebAssembly landed in Firefox 153, making it easier to work with asynchronous calls from Wasm. Here's how it works:

developer.mozilla.orgWebAssembly.Suspending - WebAssembly | MDNThe WebAssembly.Suspending object represents a suspending function — an asynchronous (Promise-based) JavaScript function that, when imported into a Wasm module and called from inside, will result in…

4 Aug

More docs: developer.mozilla.org/en-US/docs/W...

jakearchibald.comFixing my tooltip accessibility mistakearia-describedby isn't always enough.

30 July

JS text imports landed in Firefox 153, making it the first browser to support this feature. Here's how it works:

Firefox for Web Developers reposted

Jake Archibald

@jakearchibald.com

28 July

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...

youtube.comFixing my accessibility mistakeYouTube video by Firefox for Web Developers

27 July

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...

scottaohara.github.ioCustom Tooltips

27 July

Here's Scott's demos scottaohara.github.io/a11y_tooltips/, and my fixed toolbar demo random-stuff.jakearchibald.com/popover-hint...

youtube.compopover="hint" & engine diversityYouTube video by Firefox for Web Developers

23 July

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.compopover="hint" - new in Firefox 153YouTube video by Firefox for Web Developers

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)

developer.mozilla.orgUsing the Popover API - Web APIs | MDNThe Popover API provides developers with a standard, consistent, flexible mechanism for displaying popover content on top of other page content. Popover content can be controlled either declaratively ...

Full documentation on the popover API: developer.mozilla.org/en-US/docs/W...

So @tink.uk just let me know that aria-labelledby would be a better option than aria-describedby in this case. TIL!

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...

Firefox for Web Developers@webdevs.firefox.com

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