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

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

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…

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…

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/...

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.

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

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".

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:

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. …

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?

🚀 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/

❓ 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

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/...

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

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

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/

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.

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

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

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?

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?

streamHTMLUnsafe() was presented at TPAC last week. Finally, an API that lets you easily use the streaming HTML parser from JavaScript! Here's a quick overview:

Two-phase view transitions, for faster cross-document transitions, were presented at TPAC last week. Here's a quick 3min overview… https://www.youtube.com/watch?v=A6rO9RGmJgA

The W3C Security Web Application Guidelines Community Group wants to understand how developers are using specific web platform security features, and what the barriers are to the adoption. Take their survey here: https://bit.ly/3JR6JML

🚀 Firefox 145 introduces WebDriver BiDi enhancements like a new `emulation.setUserAgentOverride` command for spoofing user agents, a `browsingContext.downloadEnd` event to track completed downloads, plus various fixes. #Firefox #WebDriver

The @firefoxdevtools.bsky.social JSON viewer got better in @firefoxnightly.bsky.social : the console offers access to the original json data via `$json`. This allows to easily manipulate the underlying data to get what you want

Last week I posted about a secret config in Firefox that made view transitions easier to debug. Folks said we should make it less secret. So we did.

New in Firefox nightly (146.0a1 2025-10-19): large lists of unused CSS custom properties are collapsed by default. This not only reduces clutter, it also massively speeds up the rendering of the inspector panel in some cases.

We're currently working on devtools for view transitions in Firefox, but in the meantime, here's a little-known option that helps…

Well, isn't this handy! The font inspector in #firefox devtools links to the foundry and shows the license (tsk @ unlicensed trial 🧐) @firefoxdevtools.bsky.social #fonts

Firefox 144 now supports INP! 🦊 We just shipped interactionId, the piece that makes Interaction to Next Paint work. It helps browsers connect events from the same interaction and measure how responsive pages really feel. More here: blog.mozilla.org/performance/...

🦊 Firefox 144 adds the screen orientation and timezone override commands + the downloadWillBegin event to WebDriver BiDi! fxdx.dev/firefox-webd...

As an experiment, we (the Firefox team) wanted to try a new way to get feedback on which Interop proposals matter most. So, here's a web app where you can rank the proposals you care about, giving us data we can use when reviewing which ones to champion. interop-rank.jakearchibald.com

🦊 Firefox 143 was just released! Some WebDriver BiDi highlights: new commands to record and retrieve network responses, backfill events for `browsingContext.contextCreated` and, as usual, some bug fixes 🐛. More details at fxdx.dev/firefox-webd... !

Yay! It's the first Firefox release since I've joined the team, so let's take a look at some of the new developer-facing features in Firefox 142… 🧵

Gecko: Intent to Ship: Command and CommandFor attributes

View Transitions are enabled by default in Firefox Nightly, so they're on their way to stable. Give it a test with your current transitions, and give me a shout if anything doesn't look right.

Firefox 142 brings a few WebDriver BiDi and Classic updates: a new command to set the locale override, some changes around handling the cookies 🍪 and setting the proxy, and more at fxdx.dev/firefox-webd... 🚀

🦊 Freshly released Firefox 141 comes with a few WebDriver updates. Setup a proxy for a container with browser.createUserContext, monitor history API usage with browsingContext.historyUpdated. And also a few changes around Cookies. Read more at fxdx.dev/firefox-webd... 🚀

🦊 In Firefox 140, WebDriver BiDi allows to override certificates security settings per container and supports a new "navigationCommitted" event. This release also brings a few fixes for action simulation. More details at fxdx.dev/firefox-webd... 🚀

State of CSS 2025 is now open: stateofcss.com Browser vendors use the results of 'State of' surveys to help prioritize the features to work on. If you care about CSS, please take some time to answer it, and help shape the future of the language

The 2025 State of CSS survey is now open! survey.devographics.com/en-US/survey...

Oops, we forgot to share the WebDriver news for Firefox 138! WebDriver BiDi now lets you install unsigned extensions and allows to set a default viewport for user contexts: fxdx.dev/firefox-webd... #Firefox #WebDriver #WebDev