Firefox 156 release notes for developers (Nightly)
This article provides information about the changes in Firefox 156 that affect developers. Firefox 156 is the current Nightly version of Firefox and ships on September 15, 2026.
Note: The release notes for this Firefox version are still a work in progress.
Changes for web developers
Changes for add-on developers
- The
thememanifest key adds thebackgrounds_areaproperty. This property enables a theme to specify where its background images and gradients are drawn. Setting it to"window"draws them across the whole browser window, while"top_toolbars"restricts them to the horizontal toolbars at the top of the window. Whenbackgrounds_areais omitted or set to"auto", Firefox chooses the area based onproperties.additional_backgrounds_alignment. (Firefox bug 2059526)
Experimental web features
These features are shipping in Firefox 156 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.