This article provides information about the changes in Firefox 134 that affect developers. Firefox 134 was released onJanuary 7, 2025.

Changes for web developers

HTML

No notable changes

CSS

• Thealign-self andjustify-self CSS properties andplace-self CSS shorthand property are now supported forabsolutely positioned elements. (Firefox bug 1920160).

JavaScript

• Support for theRegExp.escape() static method that can be used to escape any potential regex syntax characters in a string, returning a new string that can be safely used as aliteral pattern for theRegExp() constructor. (Firefox bug 1918235).
• ThePromise.try() convenience method is now supported.The method takes a callback of any kind (a function that returns or throws, synchronously or asynchronously) and wraps its result in aPromise.This allows you to use promise semantics (.then(),.catch()) to handle the result from any kind of method. (Firefox bug 1917879 andFirefox bug 1905364).

APIs

• ThePushManager.supportedContentEncodings static method is now supported for getting the allowed algorithms for encrypting the payload of apush message. (Firefox bug 1497430).
• AudioParam.value now allows the value to be set even during the time that an automated event is scheduled: previously the operation would silently be ignored at those times. (Firefox bug 1308435).
• TheReadableStreamBYOBReader.read() method has a new argumentoptions.min that can be used to specify the minimum number of elements to be returned on each call. This might, for example, be used to avoid unnecessary iteration when working with data structures that have a known data size. (Firefox bug 1864406).

DOM

Media, WebRTC, and Web Audio

• WebRTC simulcast of screen-shared video with theVP8 codec is now supported (simulcast from other video sources has been enabled for a long time). More precisely,MediaStreamTrack objects for screen and window capture (for example, fromMediaDevices.getDisplayMedia()), can now be encoded as multiple simulcast layers when using VP8. (Firefox bug 1692873).

WebDriver conformance (WebDriver BiDi, Marionette)

WebDriver BiDi

• Implemented thebrowser.getClientWindows command, which allows to retrieve information about the currently opened browser windows (Firefox bug 1855025)
• Added support for theinitiatorType anddestination fields to all network events (Firefox bug 1904892 andFirefox bug 1933331). They allow to understand why and how the request was created.
• ThebrowsingContext.navigationStarted event is no longer emitted when the initial about:blank page is loaded for a new top-level browsing context (Firefox bug 1922014)
• We fixed a bug where therequestTime of network events would sometimes be set to 0 (Firefox bug 1930849)
• ThebrowsingContext.traverseHistory command can now only be used with top-level browsing contexts (Firefox bug 1924859)
• Improved the reliability of commands sent during a navigation, for instance when a browsing context is being replaced (Firefox bug 1927073).

Marionette

• TheAddon:Install andAddon:Uninstall commands are now available for GeckoView (Firefox for Android) (Firefox bug 1806135).
• TheAddon:Install command can now be used to install extensions enabled in Private Browsing mode (Firefox bug 1810718)

Experimental web features

These features are newly shipped in Firefox 134 but are disabled by default. To experiment with them, search for the appropriate preference on theabout:config page and set it totrue. You can find more such features on theExperimental features page.

• Intl.DurationFormat (Nightly release):Intl.DurationFormat enables locale-sensitive formatting of durations. (Firefox bug 1648139).
• autocorrect:dom.forms.autocorrect.The HTMLautocorrect attribute andHTMLElement.autocorrect property allow autocorrection in editable text elements, including: most kinds of text<input> elements,<textarea> elements, and elements that have thecontenteditable attribute set (Firefox bug 1725806).