Firefox 142 for developers

This article provides information about the changes in Firefox 142 that affect developers. Firefox 142 is the current Beta version of Firefox and ships on August 19, 2025.

Note: The release notes for this Firefox version are still a work in progress.

Changes for web developers

HTML

Removals

The <object> element no longer supports the deprecated codebase attribute. Use the data attribute instead. (See Firefox bug 1973900 for more details.)

APIs

DOM

The Selection.getComposedRanges() method is now supported, allowing developers to accurately get selected text ranges across shadow DOM boundaries. In addition, the methods setBaseAndExtent(), collapse(), and extend() of the Selection interface have been modified to accept nodes inside a shadow root. (Firefox bug 1903870).

Changes for add-on developers

Cookies created with cookies.set() in Nightly are now validated, and invalid cookies are rejected. The implementation in Nightly is to enable monitoring for any issues. The intention is to enforce validation in all channels in a future release. (Firefox bug 1976197)

Experimental web features

anchor-size() (Nightly): layout.css.anchor-positioning.enabled

The CSS anchor-size() function enables setting anchor-positioned element's size, position, and margins relative to the dimensions of anchor elements. (Firefox bug 1972610).
:heading and :heading(): layout.css.anchor-positioning.enabled

The CSS :heading pseudo-class allows you to style all heading elements (<h1>-<h6>) at once rather than targeting them individually. The :heading() functional pseudo-class allows you to style heading elements with the <An+B> syntax. (Firefox bug 1974386).

These features are shipping in Firefox 142 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.