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

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-...

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...

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...

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...

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...

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...

CSS contrast-color is newly baseline, but it falls short in some areas. Here's how it works, and what to look out for:

CSS contrast-color is newly baseline, but it falls short in some areas. Here's how it works, and what to look out for:

I tried this. It's hard.

hasUAVisualTransition landed in Firefox 149, making it available across all major engines. As well as having a very catchy name, it lets you avoid doubling up on page transitions. Here's how it works:

hasUAVisualTransition landed in Firefox 149, making it available across all major engines. As well as having a very catchy name, it lets you avoid doubling up on page transitions. Here's how it works:

The CloseWatcher API landed in Firefox 149, making it easy to listen for platform-specific 'dismiss' signals. Here's how it works:

Firefox 149 is out now, and adds bunch of new web platform features, like 🎉 The CloseWatcher API 🎉 popover=hint (although I'm concerned about the spec'd behaviour) 🎉 hasUAVisualTransition on PopStateEvent 🎉 font-family: math 🎉 The CSP reporting API And more! developer.mozilla.org/en-US/docs/M...

There are many browsers, but only three widely-used engines. We maintain one of those engines - Gecko. Here's why we feel that browser engine diversity is essential for the future of the web blog.mozilla.org/netpolicy/20...

Sorry everyone. You're about to spend a ton of time on this. My score is 0.0028 💅 (although I get different scores on different screens - my best is on my MacBook)

JavaScript Iterator․zip landed in Firefox 148, making it simple to loop over multiple things at the same time. Here's how it works:

CSS shape() landed in Firefox 148. It's similar to path(), but allows the full expressiveness of CSS…

WebAssembly is a second-class language on the web, but how can we make it first-class? WebAssembly Components could be the answer… hacks.mozilla.org/2026/02/maki...

You can throw away innerHTML and replace it with the new setHTML(), which has a built-in sanitizer. Here's how it works:

The Sanitizer API landed in Firefox 148, along with element.setHTML(). This lets you fully configure how HTML strings are cleaned as they're parsed. hacks.mozilla.org/2026/02/good...

Firefox 148 is out now, and adds some great web platform features: 🎉 The HTML Sanitizer API 🎉 CSS shape() 🎉 Trusted Types 🎉 Nav API's addHandler 🎉 Iterator zip & zipKeyed 🎉 ancestorOrigins And more! developer.mozilla.org/en-US/docs/M...

HTML might be getting a new type of tag, which… hasn't happened this millennium. Here's the new syntax, and how it works:

If you had to pick _one_, which web platform feature would you like in Firefox that isn't part of the Interop 2026 effort? hacks.mozilla.org/2026/02/laun...

Here's a quick rundown of what we delivered as part of Interop 2025, and what's planned for Interop 2026!

It's time for Interop 2026! This is a cross-browser effort where we agree on a set of focus areas to improve compatibility. Here's how things went in 2025, and what we're taking on for 2026: hacks.mozilla.org/2026/02/laun...

AI Controls (formally 'kill switch') are landing in today's Firefox Nightly, and will land with Firefox 148 later this month.

Here's a round-up of recent security & privacy work that went into Firefox, including: ➡️ Improvements to tracker blocking ➡️ Integrity-Policy header support ➡️ Compressed Elliptic Curve Points in WebCrypto attackanddefense.dev/2026/01/30/f...

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. …