JS promise integration for WebAssembly landed in Firefox 153, making it easier to work with asynchronous calls from Wasm. Here's how it works:
Firefox for Web Developers
@webdevs.firefox.com
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.
JS text imports landed in Firefox 153, making it the first browser to support this feature. Here's how it works:
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...
Fixing my tooltip accessibility mistake
aria-describedby isn't always enough.
jakearchibald.com
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...
Fixing my accessibility mistake
YouTube video by Firefox for Web Developers
youtube.com
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...
popover="hint" & engine diversity
YouTube video by Firefox for Web Developers
youtube.com
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)
popover="hint" - new in Firefox 153
YouTube video by Firefox for Web Developers
youtube.com
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 · 8mo agoJavaScript declarations in conditionals was discussed at TC39 recently, but consensus broke down over one key detail. How do you think it should behave?
Firefox 153 just landed, and comes with web platform features like: 🎉 popover="hint" 🎉 Picture-in-picture 🎉 IDB getAllRecords() 🎉 JS Promise Integration for wasm 🎉 JS import-as-text …And more! developer.mozilla.org/en-US/docs/M...
Firefox 153 release notes for developers (Stable) - Mozilla | MDN
This article provides information about the changes in Firefox 153 that affect developers. Firefox 153 was released on July 21, 2026.
developer.mozilla.org
Someone said our Instagram page looks like "the world's most boring game of Guess Who" 😭 www.instagram.com/firefoxwebde...
Notification actions landed in Firefox 152, allowing you to add quick action buttons to your notifications. Here's how they work:
CSS field-sizing is newly baseline, meaning you can make various form fields size to their contents, without needing JavaScript.
Firefox 152 landed today, and includes web platform features like: 🎉 CSS field-sizing 🎉 Notification actions 🎉 unadjustedMovement pointer locks 🎉 getAnimations() pseudoElement option …and more! developer.mozilla.org/en-US/docs/M...
Firefox 152 release notes for developers (Beta) - Mozilla | MDN
This article provides information about the changes in Firefox 152 that affect developers. Firefox 152 is the current Beta version of Firefox and ships on June 16, 2026.
developer.mozilla.org
If your content is served over HTTP/3, make sure your DNS record advertises it, otherwise you're missing out on free performance. Check your site here: savearoundtrip.com
savearoundtrip
Publish an HTTPS DNS record. Let browsers reach HTTP/3 on the first connection instead of wasting a round trip.
savearoundtrip.com
Ever struggled to get heading tags right in includes/components? headingoffset could be the answer…
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...
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:
Which of the following three CSS features would you prefer Firefox to support first? ❤️ your favorite in the thread:
Web Serial landed in Firefox 151! Here's how it works, and what you can do with it: hacks.mozilla.org/2026/05/web-...
Announcing Web Serial Support in Firefox – Mozilla Hacks - the Web developer blog
Support for Web Serial in Firefox 151 for Desktop Firefox can now connect directly to microcontrollers, development boards, 3D printers, power meters, and other serial-connected hardware from the…
hacks.mozilla.org
CSS style() queries landed in Firefox 151 making them newly available across engines. Here's how they work:
Firefox 151 is out now, and brings with it some tasty new web platform features: 🎉 Container style() queries 🎉 Web Serial API 🎉 Document Picture-in-Picture API 🎉 keyboardLock on requestFullscreen …and more! developer.mozilla.org/en-US/docs/M...
Firefox 151 release notes for developers (Stable) - Mozilla | MDN
This article provides information about the changes in Firefox 151 that affect developers. Firefox 151 was released on May 19, 2026.
developer.mozilla.org
Chrome shipped an LLM Prompt API to the web platform. At Mozilla, we oppose this API. Here's why:
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/behi...
Behind the Scenes Hardening Firefox with Claude Mythos Preview – Mozilla Hacks - the Web developer blog
New details about what we found, and how agentic harnesses are now able to reproduce real bugs and dismiss false positives.
hacks.mozilla.org
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: github.com/mozilla/stan...
Prompt API · Issue #1213 · mozilla/standards-positions
Specification title Prompt API Specification or proposal URL (if available) No response Explainer URL (if available) https://github.com/webmachinelearning/prompt-api/blob/main/README.md Proposal au...
github.com
One thing the example with the `@media` query doesn't quite capture (and I know this is meant to be a quick 1 min video), is that `light-dark()` will respect a "local" `color-scheme` override as well. So it's more than just a shorthand for the media query!
CSS's light-dark() now accepts images, making it simpler to define alternate values for a dark color scheme. Here's how it works:
sizes="auto" landed in Firefox 150, making responsive images simpler and more reliable. Here's when you can use it, and how…
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/M...
Firefox 150 release notes for developers (Stable) - Mozilla | MDN
This article provides information about the changes in Firefox 150 that affect developers. Firefox 150 was released on April 21, 2026.
developer.mozilla.org
map.getOrInsertComputed is newly baseline! It's a small feature, but I've found it pretty useful. Here's how it works:
A common mistake with a 'subtree' MutationObserver is assuming addedNodes includes all newly connected nodes, but it doesn't include preexisting children of the added nodes codepen.io/editor/jaffa...
Mutation observer gotcha
A code demo by Jake Archibald created on CodePen
codepen.io