Firefox for Web Developers

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/Moz…

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/behind…

@Rok_Kralj We wonder if it might be related to bugzilla.mozilla.org/show_bug.cgi?i…, but looking at the URL would really help us here.

@Rok_Kralj Is the page you're seeing it on public? Can I get the URL?

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/Moz…

@TLRailUK @ndabas When we land code, the process of it going through Nightly & Beta, to Release, is called "riding the trains", so this is on brand for us. firefox-source-docs.mozilla.org/contributing/p…

@ndabas Interesting ! That is a new one. Please could you tell me which train you are on (ie 1020 from London Bridge to Brighton) and also which coach you are in ? ^Kieran

I didn’t know @TLRailUK used web pages and Firefox for the PIS!

@Rok_Kralj Is this error reliably reproducible, or is it intermittent?

@FirefoxWebDevs This was never observed before. And it is literally impossible to happen, because we emit headers and response body in Netty in a single ByteBuf. Header on root HTML document is "Cache-Control: private,max-age=1209600". Did you make any changes to caching in FF 150?

@scottdotnetdev go have a look and find out

@FirefoxWebDevs Is this in there? x.com/i/status/20432…

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: #issuecomment-4347988313" target="_blank" rel="nofollow noopener">github.com/mozilla/standa…

@TweetingTweet01 Folks are working on a response to that. Keep an eye on the issue.

@FirefoxWebDevs What about WebMCP? This time it's not model specific and it's actually the other way around, for AIs to access tools in web pages and not pages accessing AIs. github.com/mozilla/standa…

@scottdotnetdev well someone hasn't been reading the @FirefoxWebDevs release notes

@FirefoxWebDevs You have 1% browser marketshare and haven't had an innovation in a decade. At the very least don't be an impediment to the ones that are.

@bashln It's not an area I work on personally, but I'll ping people internally about it

@FirefoxWebDevs Could you give us some love on this topic from Mozilla Connect? connect.mozilla.org/t5/ideas/synce…

CSS's light-dark() now accepts images, making it simpler to define alternate values for a dark color scheme. Here's how it works:

@raddevus Nope, new versions have been going out every four weeks. The most recent was on Tuesday x.com/FirefoxWebDevs… Edge and Brave both use Chrome's rendering engine. Firefox has an independent engine.

@raddevus I think you may have it on _bad_ authority.

@ShirtlessKirk Yesss! You can also do sizes="auto, …all the usual shit" and older browsers will ignore the 'auto'.

@FirefoxWebDevs Bonus being that if you use `<source>` tags with your `<img>` all wrapped in a `<picture>`, as I do, you can still add `sizes="auto"` to the lazy loaded `<img>` tag as that value will be ignored by the older browsers you still have to support.

sizes="auto" landed in Firefox 150, making responsive images simpler and more reliable. Here's when you can use it, and how…

@jens023 Aww thank you!

@FirefoxWebDevs It's a pleasure listening to you!

@adamjosefus Thank you! I'm glad these are landing well. They're fun to make!

@FirefoxWebDevs You're so good at simplifying web "chaos" into just a few words. Thank you so much, Jake!

There are loads more attributes you can use to control how images are loaded and rendered. Here are the full docs #attributes" target="_blank" rel="nofollow noopener">developer.mozilla.org/en-US/docs/Web…

@Una @rdeschain4 Saaaaame!

@FirefoxWebDevs @rdeschain4 Can't wait for style queries in 151 :)

@rdeschain4 @Una Full steam ahead! Follow along at bugzilla.mozilla.org/show_bug.cgi?i…. There's no specific eta, but it's being actively worked on.

@FirefoxWebDevs @Una How far along is customizable select?