Firefox 139 for developers

This article provides information about the changes in Firefox 139 that affect developers. Firefox 139 is the current Nightly version of Firefox and ships on May 27, 2025.

Changes for web developers

Developer Tools

HTML

Removals

CSS

Removals

JavaScript

Removals

SVG

Removals

HTTP

Removals

Security

Removals

APIs

DOM

Media, WebRTC, and Web Audio

Removals

WebAssembly

Removals

WebDriver conformance (WebDriver BiDi, Marionette)

General

WebDriver BiDi

Marionette

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 (such as en-GB or zh-Hans-CN). Take an extension with the default of Spanish (es) and a translation for English (en) installed by a user who has chosen British English (en-GB) as their browser locale. Previously, the user was served the default (Spanish) translation. After this change, the user is served the English (en) translation. (Firefox bug 1381580)

Removals

Other

Experimental web features

These features are newly 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.

Older versions