What Changed in Gecko: Firefox 133
GeckoView EngineWhy This Matters
Gecko is the last truly independent browser engine — the only major alternative to Google’s Blink. Every Firefox release ripples through Tor Browser, LibreWolf, Mullvad Browser, and the broader privacy-focused ecosystem. Firefox 133 is a privacy-forward release: Bounce Tracking Protection debuts, Fetch keepalive arrives, and developers get Uint8Array Base64/hex methods plus WebCodecs image decoding. Here’s the full picture.
JavaScript
Uint8Array Base64 and hex methods are the JavaScript headline. 6 new methods — fromBase64(), fromHex(), setFromBase64(), setFromHex(), toBase64(), and toHex() — bring native encoding/decoding to typed arrays without relying on btoa/atob workarounds (bug 1917885). This is a TC39 Stage 3 proposal that eliminates a common pain point for binary data handling.
CSS
No notable CSS changes shipped in 133. Behind experimental flags, the :has-slotted pseudo-class is available for styling slotted content in web components (bug 1921747), and contenteditable="plaintext-only" can be tested for rich-text-free editing (bug 1922723).
Web APIs
Fetch keepalive is the most practical API addition. When set to true, the browser won’t abort a fetch request even if the page unloads — perfect for analytics pings and session-end beacons. It works as a modern replacement for Navigator.sendBeacon() with more flexibility: any HTTP method, custom headers, and access to the response Promise (bug 1906952).
Dialog beforetoggle and toggle events now fire on <dialog> elements, matching popover behavior. Use beforetoggle to reset form state before a dialog opens, and toggle to react after it closes (bug 1876762).
The WebCodecs ImageDecoder API ships, enabling image decoding from both main and worker threads — a building block for high-performance image processing pipelines (bug 1923755).
The Permissions API now works in Worker contexts (bug 1193373), and EventSource (Server-Sent Events) is supported in service workers (bug 1681218). ServiceWorkerContainer is also now exposed in all worker contexts via WorkerNavigator.serviceWorker (bug 1113522).
The interactive-widget viewport meta attribute is now supported, giving developers control over how virtual keyboards and other UI widgets affect the viewport size (bug 1831649).
Security & Privacy
The marquee feature is Bounce Tracking Protection, now available in Enhanced Tracking Protection’s Strict mode. It detects trackers that use redirect chains to set cookies, then periodically purges their data. This targets a tracking technique that traditional cookie-blocking misses — a significant upgrade to Firefox’s privacy arsenal.
Performance
Canvas2D acceleration on Windows switched from Direct2D to a platform-independent backend, improving consistency and maintainability across operating systems (Firefox release notes).
The Picture-in-Picture auto-open feature (from Firefox Labs) now works more reliably across sites, correctly identifying which videos to float and which to ignore.
Cookie expiration handling improved: when server time is available, the expire attribute is now adjusted for clock differences between server and client, preventing premature cookie invalidation.
Developer Tools
WebDriver BiDi gained the ability to redirect requests via the url argument in network.continueRequest (bug 1898158). Marionette improved Wayland window positioning on Linux (bug 1857571) and now includes alert text in UnexpectedAlertOpen error messages (bug 1924469).
For extension developers, cookies.get now orders results per RFC 6265, returning the longest matching path first instead of the earliest created cookie (bug 1798655).
Bottom Line
Web developers get Fetch keepalive, Uint8Array encoding methods, and WebCodecs ImageDecoder — all practical tools that reduce dependency on third-party libraries. Privacy advocates should celebrate Bounce Tracking Protection, which closes a significant gap in Firefox’s anti-tracking defenses. Service worker authors benefit from EventSource support and the expanded Permissions API. The entire Gecko ecosystem — Tor, LibreWolf, Mullvad — inherits a meaningfully more private and capable engine.