Firefox for Web Developers
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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 147 release notes for developers (Beta) - Mozilla | MDN
This article provides information about the changes in Firefox 147 that affect developers.
Firefox 147 is the current Beta version of Firefox and ships on January 13, 2026.
developer.mozilla.org
Firefox 147 just landed & it's pretty huge in terms of web features:
🎉 CSS anchor positioning
🎉 The navigation API
🎉 View transition types
🎉 Brotli support in Compression/DecompressionStream
🎉 CSS module imports
And more!
https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/147
🎉 CSS anchor positioning
🎉 The navigation API
🎉 View transition types
🎉 Brotli support in Compression/DecompressionStream
🎉 CSS module imports
And more!
https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/147
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…
Also, we have some new View Transition DevTools in Firefox 147…
January 19, 2026 at 2:13 PM
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…
Also, we have some new View Transition DevTools in Firefox 147…
Reposted by Firefox for Web Developers
sign() is Newly Available on Baseline ➖➕
Unlike abs(), which returns a magnitude, sign() returns the direction of a value: -1, 1, 0, or -0.
Useful when layout logic depends on whether a value is positive or negative.
Learn more here 👇
developer.mozilla.org/en-US/docs/...
Unlike abs(), which returns a magnitude, sign() returns the direction of a value: -1, 1, 0, or -0.
Useful when layout logic depends on whether a value is positive or negative.
Learn more here 👇
developer.mozilla.org/en-US/docs/...
January 16, 2026 at 4:00 AM
sign() is Newly Available on Baseline ➖➕
Unlike abs(), which returns a magnitude, sign() returns the direction of a value: -1, 1, 0, or -0.
Useful when layout logic depends on whether a value is positive or negative.
Learn more here 👇
developer.mozilla.org/en-US/docs/...
Unlike abs(), which returns a magnitude, sign() returns the direction of a value: -1, 1, 0, or -0.
Useful when layout logic depends on whether a value is positive or negative.
Learn more here 👇
developer.mozilla.org/en-US/docs/...
The Navigation API is newly baseline! The web now has sensible, low-level routing for navigations.
January 15, 2026 at 3:25 PM
The Navigation API is newly baseline! The web now has sensible, low-level routing for navigations.
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.
It's a huge feature, but here are the basics.
January 13, 2026 at 4:09 PM
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.
It's a huge feature, but here are the basics.
Firefox 147 just landed & it's pretty huge in terms of web features:
🎉 CSS anchor positioning
🎉 The navigation API
🎉 View transition types
🎉 Brotli support in Compression/DecompressionStream
🎉 CSS module imports
And more!
https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/147
🎉 CSS anchor positioning
🎉 The navigation API
🎉 View transition types
🎉 Brotli support in Compression/DecompressionStream
🎉 CSS module imports
And more!
https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/147
Firefox 147 release notes for developers (Beta) - Mozilla | MDN
This article provides information about the changes in Firefox 147 that affect developers.
Firefox 147 is the current Beta version of Firefox and ships on January 13, 2026.
developer.mozilla.org
January 13, 2026 at 2:03 PM
Firefox 147 just landed & it's pretty huge in terms of web features:
🎉 CSS anchor positioning
🎉 The navigation API
🎉 View transition types
🎉 Brotli support in Compression/DecompressionStream
🎉 CSS module imports
And more!
https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/147
🎉 CSS anchor positioning
🎉 The navigation API
🎉 View transition types
🎉 Brotli support in Compression/DecompressionStream
🎉 CSS module imports
And more!
https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/147
TIL that:
<? this ?>
<! this !>
</ and this >
…all parse as HTML comments.
It hits a part of the parser the spec where the content is "reconsumed in the bogus comment state".
<? this ?>
<! this !>
</ and this >
…all parse as HTML comments.
It hits a part of the parser the spec where the content is "reconsumed in the bogus comment state".
January 9, 2026 at 12:45 PM
TIL that:
<? this ?>
<! this !>
</ and this >
…all parse as HTML comments.
It hits a part of the parser the spec where the content is "reconsumed in the bogus comment state".
<? this ?>
<! this !>
</ and this >
…all parse as HTML comments.
It hits a part of the parser the spec where the content is "reconsumed in the bogus comment state".
TIL Firefox DevTools console has a $$$('selector') command that returns an array of matching elements, including those in shadow roots.
January 6, 2026 at 9:42 AM
TIL Firefox DevTools console has a $$$('selector') command that returns an array of matching elements, including those in shadow roots.
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:
December 19, 2025 at 1:45 PM
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:
Something that hasn't been made clear: Firefox will have an option to completely disable all AI features.
We've been calling it the AI kill switch internally. I'm sure it'll ship with a less murderous name, but that's how seriously and absolutely we're taking this.
…
We've been calling it the AI kill switch internally. I'm sure it'll ship with a less murderous name, but that's how seriously and absolutely we're taking this.
…
December 18, 2025 at 12:12 PM
Something that hasn't been made clear: Firefox will have an option to completely disable all AI features.
We've been calling it the AI kill switch internally. I'm sure it'll ship with a less murderous name, but that's how seriously and absolutely we're taking this.
…
We've been calling it the AI kill switch internally. I'm sure it'll ship with a less murderous name, but that's how seriously and absolutely we're taking this.
…
HTML invoker commands are newly baseline! Here's how they work…
December 17, 2025 at 3:58 PM
HTML invoker commands are newly baseline! Here's how they work…
JavaScript declarations in conditionals was discussed at TC39 recently, but consensus broke down over one key detail. How do you think it should behave?
December 15, 2025 at 3:19 PM
JavaScript declarations in conditionals was discussed at TC39 recently, but consensus broke down over one key detail. How do you think it should behave?
🚀 A bunch of #WebDriver features and fixes landed in Firefox 146. WebDriver is the W3C standard for interoperable browser automation, for things like testing tools, and MCPs.
Keep up to date with all the progress here:
https://fxdx.dev/firefox-webdriver-newsletter-146/
Keep up to date with all the progress here:
https://fxdx.dev/firefox-webdriver-newsletter-146/
Firefox WebDriver Newsletter 146 – Firefox Developer Experience
fxdx.dev
December 15, 2025 at 12:47 PM
🚀 A bunch of #WebDriver features and fixes landed in Firefox 146. WebDriver is the W3C standard for interoperable browser automation, for things like testing tools, and MCPs.
Keep up to date with all the progress here:
https://fxdx.dev/firefox-webdriver-newsletter-146/
Keep up to date with all the progress here:
https://fxdx.dev/firefox-webdriver-newsletter-146/
❓ Do you have a need for range inputs with 3+ sliders on the web? Folks are scoping out this work, and want to know your use-cases ⬇️ https://github.com/openui/open-ui/issues/1337
December 12, 2025 at 3:30 PM
❓ Do you have a need for range inputs with 3+ sliders on the web? Folks are scoping out this work, and want to know your use-cases ⬇️ https://github.com/openui/open-ui/issues/1337
In Firefox 146 DevTools, long lists of unused CSS custom properties collapse, decluttering the UI, and improving performance
December 12, 2025 at 9:50 AM
In Firefox 146 DevTools, long lists of unused CSS custom properties collapse, decluttering the UI, and improving performance
Reposted by Firefox for Web Developers
Fine-tune multilingual typography with text-autospace ✨
It lets you control spacing between CJK and Latin characters.
> text-autospace: ideograph-alpha ideograph-numeric punctuation;
Now available on Baseline 2025. 🎉
Learn more 👇
developer.mozilla.org/en-US/docs/...
It lets you control spacing between CJK and Latin characters.
> text-autospace: ideograph-alpha ideograph-numeric punctuation;
Now available on Baseline 2025. 🎉
Learn more 👇
developer.mozilla.org/en-US/docs/...
December 11, 2025 at 12:00 PM
Fine-tune multilingual typography with text-autospace ✨
It lets you control spacing between CJK and Latin characters.
> text-autospace: ideograph-alpha ideograph-numeric punctuation;
Now available on Baseline 2025. 🎉
Learn more 👇
developer.mozilla.org/en-US/docs/...
It lets you control spacing between CJK and Latin characters.
> text-autospace: ideograph-alpha ideograph-numeric punctuation;
Now available on Baseline 2025. 🎉
Learn more 👇
developer.mozilla.org/en-US/docs/...
CSS text-decoration-inset landed in Firefox 146! Here's how it works:
December 11, 2025 at 5:11 PM
CSS text-decoration-inset landed in Firefox 146! Here's how it works:
A bunch of web platform features and bug fixes landed in Firefox 146. There'll be more videos about the highlights soon, but in the meantime, here's a summary: https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/146
Firefox 146 release notes for developers (Stable) - Mozilla | MDN
This article provides information about the changes in Firefox 146 that affect developers.
Firefox 146 was released on December 9, 2025.
developer.mozilla.org
December 10, 2025 at 11:56 AM
A bunch of web platform features and bug fixes landed in Firefox 146. There'll be more videos about the highlights soon, but in the meantime, here's a summary: https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/146
CSS @scope has landed in Firefox 146, making it a new baseline feature. Here's how it works:
December 9, 2025 at 3:55 PM
CSS @scope has landed in Firefox 146, making it a new baseline feature. Here's how it works:
Reposted by Firefox for Web Developers
New blog post: Why the Sanitizer API is just `setHTML()` - https://frederikbraun.de/why-sethtml.html
December 7, 2025 at 9:14 PM
New blog post: Why the Sanitizer API is just `setHTML()` - https://frederikbraun.de/why-sethtml.html
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.
https://blog.mozilla.org/en/mozilla/av1-video-codec-wins-emmy/
https://blog.mozilla.org/en/mozilla/av1-video-codec-wins-emmy/
December 5, 2025 at 4:59 PM
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.
https://blog.mozilla.org/en/mozilla/av1-video-codec-wins-emmy/
https://blog.mozilla.org/en/mozilla/av1-video-codec-wins-emmy/
Oh hello! We've had a bit of a rename & rebrand (formally FirefoxDevTools), but the content will be roughly the same as it's been for the past few months.
We're looking forward to posting more on web platform & DevTools features.
We're looking forward to posting more on web platform & DevTools features.
December 5, 2025 at 1:38 PM
Oh hello! We've had a bit of a rename & rebrand (formally FirefoxDevTools), but the content will be roughly the same as it's been for the past few months.
We're looking forward to posting more on web platform & DevTools features.
We're looking forward to posting more on web platform & DevTools features.
Another API proposal from TPAC: CSS routes and navigation queries, meaning less JavaScript is needed for view transitions!
https://drafts.csswg.org/css-navigation-1/
https://github.com/w3c/csswg-drafts/issues/8685#issue-1653949919
https://drafts.csswg.org/css-navigation-1/
https://github.com/w3c/csswg-drafts/issues/8685#issue-1653949919
December 2, 2025 at 4:02 PM
Another API proposal from TPAC: CSS routes and navigation queries, meaning less JavaScript is needed for view transitions!
https://drafts.csswg.org/css-navigation-1/
https://github.com/w3c/csswg-drafts/issues/8685#issue-1653949919
https://drafts.csswg.org/css-navigation-1/
https://github.com/w3c/csswg-drafts/issues/8685#issue-1653949919
We have Custom Elements, but do we also need Custom Attributes?
This was discussed at TPAC. Is it something you'd like on the platform?
https://github.com/WICG/webcomponents/issues/1029
This was discussed at TPAC. Is it something you'd like on the platform?
https://github.com/WICG/webcomponents/issues/1029
December 1, 2025 at 2:29 PM
We have Custom Elements, but do we also need Custom Attributes?
This was discussed at TPAC. Is it something you'd like on the platform?
https://github.com/WICG/webcomponents/issues/1029
This was discussed at TPAC. Is it something you'd like on the platform?
https://github.com/WICG/webcomponents/issues/1029
Reposted by Firefox for Web Developers
We (@firefoxdevtools.bsky.social) are having a work week with the folks from @developer.mozilla.org to talk about better collaboration between our teams.
We already have some ideas that we'll share in the future, but it would I'm curious if you have ideas around that topic?
We already have some ideas that we'll share in the future, but it would I'm curious if you have ideas around that topic?
November 25, 2025 at 10:52 AM
We (@firefoxdevtools.bsky.social) are having a work week with the folks from @developer.mozilla.org to talk about better collaboration between our teams.
We already have some ideas that we'll share in the future, but it would I'm curious if you have ideas around that topic?
We already have some ideas that we'll share in the future, but it would I'm curious if you have ideas around that topic?
Out-of-order HTML patching, another proposal from TPAC, lets HTML be presented non-linearly, and a single file can update multiple parts of the page.
Would this feature be useful to you?
Would this feature be useful to you?
November 19, 2025 at 4:10 PM
Out-of-order HTML patching, another proposal from TPAC, lets HTML be presented non-linearly, and a single file can update multiple parts of the page.
Would this feature be useful to you?
Would this feature be useful to you?