This article provides information about the changes in Firefox 137 that affect developers.Firefox 137 was released onApril 1, 2025.

Changes for web developers

HTML

No notable changes

CSS

• Thehyphenate-limit-chars CSS property provides you with fine-grained control over hyphenation in text. It is used to specify the minimum word length for hyphenation as well as the number of characters before and after the hyphen. (Firefox bug 1947183).
• Thetext-decoration-line CSS property now accepts the valuesspelling-error andgrammar-error. These values use the browser's styling for spelling and grammar errors and disregard the other properties in thetext-decoration shorthand. (Firefox bug 1950844).

JavaScript

• TheMath.sumPrecise() static method is now supported. This takes aniterable (such as anArray) of numbers and returns their sum. It is more precise than summing the numbers in a loop because it avoids floating point precision loss in intermediate results. (Firefox bug 1943120).
• TheAtomics.pause() static method is now supported. This method provides a hint to the CPU that the current thread is in a spinlock while waiting on access to a shared resource. The system can then reduce the resources allocated to the core (such as power) or thread, without yielding the current thread. (Firefox bug 1937805).

APIs

• ThegetPathData(),setPathData(), andgetPathSegmentAtLength() methods of theSVGPathElement interface are now supported. These methods provide a convenient way to work with SVG path data instead of parsing raw string data. (Firefox bug 1945312).

Media, WebRTC, and Web Audio

• HEVC (H.265) is now hardware enabled on Android, and hardware and software enabled on Linux. This adds to existing hardware and software support on Windows and macOS. (Firefox bug 1950032).

Removals

• The following non-standard events are now deprecated and proposed for removal:afterscriptexecute andbeforescriptexecute inDocument, andafterscriptexecute, andbeforescriptexecute inElement. A console warning is displayed when they are used. (Firefox bug 1949373).

WebDriver conformance (WebDriver BiDi, Marionette)

General

• Input sources of typemouse andtouch now support fractional numbers for x and y positions for thepointerMove action (Firefox bug 1946774).

WebDriver BiDi

• Implemented newwebExtension.install (Firefox bug 1934551) andwebExtension.uninstall (Firefox bug 1934553) commands, enabling clients to install and uninstall web extensions in the browser.
• Added support foruserContexts argument tosessions.subscribe command, which allows clients subscribing to a list of user contexts (also known as Firefox containers)(Firefox bug 1938604).
• script.addPreloadScript command is updated to throw aninvalid argument error when bothcontexts anduserContexts arguments are provided (Firefox bug 1945554).
• browsingContext.navigate command will not return immediately anymore whenwait argument equalsnone and beforeunload prompt opens. That's the first step of updating the logic behindwait argument equalsnone (Firefox bug 1948700).

Marionette

• From now on, ajavascript error from a syntax error raised by script evaluation will contain line and column number (Firefox bug 1865146).
• Performing actions with async events enabled will not fail anymore with aCyclic object value error message (Firefox bug 1947112).Async events were enabled since Firefox 135. See more details inthe release notes.

Changes for add-on developers

• Addscommands.openShortcutSettings that opens the Manage Extension Shortcuts page of Manage Your Extensions (about:addons) and, if the extension has shortcuts, scrolls to and highlights the extension's shortcut key options. (Firefox bug 1538451)
• The 10 MB quota for data stored by thestorage.session API is now enforced. (Firefox bug 1915688)

Experimental web features

These features are newly shipped in Firefox 137 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.

• Temporal API (Nightly release): is now enabled in Firefox Nightly by default. TheTemporal object aims to simplify working with dates and times in various scenarios, with built-in time zone and calendar representations. (Firefox bug 1946823).