Firefox 137 release notes for developers

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

Changes for web developers

HTML

No notable changes

CSS

• The {{CSSXRef("hyphenate-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).
• The {{cssxref("text-decoration-line")}} CSS property now accepts the values spelling-error and grammar-error. These values use the browser's styling for spelling and grammar errors and disregard the other properties in the {{cssxref("text-decoration")}} shorthand. (Firefox bug 1950844).

JavaScript

• The {{jsxref("Math.sumPrecise()")}} static method is now supported. This takes an iterable (such as an {{jsxref("Array")}}) 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).
• The {{jsxref("Atomics.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

• The {{domxref("SVGPathElement/getPathData", "getPathData()")}}, {{domxref("SVGPathElement/setPathData", "setPathData()")}}, and {{domxref("SVGPathElement/getPathSegmentAtLength", "getPathSegmentAtLength()")}} methods of the {{domxref("SVGPathElement")}} 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 and beforescriptexecute in {{domxref("Document")}}, and afterscriptexecute, and beforescriptexecute in {{domxref("Element")}}. A console warning is displayed when they are used. (Firefox bug 1949373).

WebDriver conformance (WebDriver BiDi, Marionette)

General

• Input sources of type mouse and touch now support fractional numbers for x and y positions for the pointerMove action (Firefox bug 1946774).

WebDriver BiDi

• Implemented new webExtension.install (Firefox bug 1934551) and webExtension.uninstall (Firefox bug 1934553) commands, enabling clients to install and uninstall web extensions in the browser.
• Added support for userContexts argument to sessions.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 an invalid argument error when both contexts and userContexts arguments are provided (Firefox bug 1945554).
• browsingContext.navigate command will not return immediately anymore when wait argument equals none and beforeunload prompt opens. That's the first step of updating the logic behind wait argument equals none (Firefox bug 1948700).

Marionette

• From now on, a javascript 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 a Cyclic object value error message (Firefox bug 1947112). Async events were enabled since Firefox 135. See more details in the release notes.

Changes for add-on developers

• Adds {{WebExtAPIRef("commands.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 the {{WebExtAPIRef("storage.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 the about:config page and set it to true. You can find more such features on the Experimental features page.

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