Firefox 139 release notes for developers - Content

This article provides information about the changes in Firefox 139 that affect developers. Firefox 139 was released on May 27, 2025.

Changes for web developers

HTML

The hidden=until-found HTML attribute and the beforematch event are now supported. The hidden until found state allows you to hide the contents of an element until it is found by user search (for example, using "Find in page") or by fragment navigation. The beforematch event fires just before the hidden attribute is removed (Firefox bug 1761043 and Firefox bug 1955379).

CSS

No notable changes

JavaScript

The Temporal API is now supported, this aims to simplify working with dates and times in various scenarios, with built-in time zone and calendar representations (Firefox bug 1912511 and Firefox bug 1954138). This includes:
- A duration (difference between two time points): {{jsxref("Temporal.Duration")}}
- Points in time:
  - As a unique instant in history:
    - A timestamp: {{jsxref("Temporal.Instant")}}
    - A date-time with a time zone: {{jsxref("Temporal.ZonedDateTime")}}
  - Time-zone-unaware date/time ("Plain"):
    - Date (year, month, day) + time (hour, minute, second, millisecond, nanosecond): {{jsxref("Temporal.PlainDateTime")}}
      - Date (year, month, day): {{jsxref("Temporal.PlainDate")}}
        
        Year, month: {{jsxref("Temporal.PlainYearMonth")}}
        
        Month, day: {{jsxref("Temporal.PlainMonthDay")}}
      - Time (hour, minute, second, millisecond, nanosecond): {{jsxref("Temporal.PlainTime")}}
- Now (current time) as various class instances, or in a specific format: {{jsxref("Temporal.Now")}}
Currently the following calendar types are disabled for the withCalendar() method for PlainDate, PlainDateTime, and ZonedDateTime objects:
- islamic
- islamic-rgsa
- islamic-umalqura

SVG

Removals

The experimental <discard> element has been removed, along with its corresponding {{domxref("SVGDiscardElement")}} JavaScript interface. These aren't supported in other browsers and are expected to be removed from the specification. (Firefox bug 1958839).

HTTP

No notable changes

APIs

DOM

The {{domxref("HTMLDialogElement/requestClose", "requestClose()")}} method of the {{domxref("HTMLDialogElement")}} interface is now supported. This allows developers to conditionally prevent a dialog from closing by providing a cancel event handler. (Firefox bug 1960556).
The largeBlob and credProps Web Authentication extensions are now supported, while the prf extension is supported on all desktop platforms. These respectively provide support for large blob storage associated with a credential, additional information about a credential, and a mechanism for generating a random number associated with a credential and a particular input. (Firefox bug 1795020, Firefox bug 1844449, Firefox bug 1935280).

WebDriver conformance (WebDriver BiDi, Marionette)

General

The priority manager (on macOS, the background QoS manager) in Firefox, which manages IPC message prioritization between the parent and content processes, has been temporarily disabled for all supported Remote Protocols. This change resolves an issue where, under high system load, initial page loads in newly opened background tabs were not being scheduled, resulting in certain commands hanging (Firefox bug 1960734).

WebDriver BiDi

Implemented the emulation.setGeolocationOverride command, allowing tests and automation tools to simulate geographic locations across specified browsing contexts or user contexts. This enables consumers to test location-aware features such as geofencing for local recommendations (Firefox bug 1954992).

Changes for add-on developers

Localized extensions now cascade through locale subtags to find translations before reverting to the extension's default language. Previously, the extension used the extension default if a translation couldn't be found for a language with subtags. See Localized string selection in the Internationalization article for more details of the new behavior. (Firefox bug 1381580)
Content scripts and styles are now guaranteed to execute in the order of registration (i.e., their order in the content_scripts manifest key array). Previously, the order was only guaranteed for scripts within the same js array. (Firefox bug 1792685)
The {{WebExtAPIRef("tabGroups")}} API is now available. This API enables extensions to modify and rearrange tab groups. For more information, see WebExtensions Support for Tab Groups. (Firefox bug 1940631)

Experimental web features

These features shipped in Firefox 139 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.

Prioritized Task Scheduling API (Nightly release). The Prioritized Task Scheduling API provides a standardized way to prioritize all tasks belonging to an application, whether they are defined in a website developer's code, or in third-party libraries and frameworks. This adds support for the {{domxref("scheduler.yield()")}} method and re-enables the whole API in the Nightly release. (Firefox bug 1958943, Firefox bug 1920115).
View Transition API (Nightly release). The View Transition API has been enabled for SPAs (single-page applications). It provides a mechanism for easily creating animated transitions between different website views. (Firefox bug 1950759).
Support for escaping < and > in attributes when serializing HTML: dom.security.html_serialization_escape_lt_gt. Firefox now replaces the < and > characters with < and >, respectively, in attributes when serializing HTML. This helps prevent certain exploits where HTML is serialized and then injected back into the DOM. The affected methods and properties are: {{domxref("Element.innerHTML")}}, {{domxref("Element.outerHTML")}}, {{domxref("Element.getHTML()")}}, {{domxref("ShadowRoot.innerHTML")}}, and {{domxref("ShadowRoot.getHTML()")}}. (Firefox bug 1941347).
Disable non-standard beforescriptexecute and afterscriptexecute: dom.events.script_execute.enabled. The events have been disabled on Nightly only, allowing browser testing prior to their removal. The affected events are: beforescriptexecute and afterscriptexecute on the {{domxref("Document")}} interface, and afterscriptexecute and beforescriptexecute on the {{domxref("Element")}} interface. (Firefox bug 1954685).