Firefox 146 release notes for developers (Nightly)
This article provides information about the changes in Firefox 146 that affect developers. Firefox 146 is the current Nightly version of Firefox and ships on December 9, 2025.
Note: The release notes for this Firefox version are still a work in progress.
Changes for web developers
CSS
- The legacy
-webkit-fill-availablekeyword is now supported as a value for the CSSwidthandheightproperties to improve web compatibility. This keyword is an alias for the recently-standardizedstretchkeyword (i.e.,width: stretchandheight: stretch), which isn't yet supported in Firefox. (Firefox bug 1988938, Firefox bug 1789477).
Changes for add-on developers
Experimental web features
These features are shipping in Firefox 146 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.