Firefox 142 for developers

Removals

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

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).
• The Animation.overallProgress property is now supported, allowing developers to track and display progress through an animation. (Firefox bug 1834878).
• The Animation.commitStyles() method no longer requires fill to be set on an animation to commit the computed styles after the animation has finished. Note that until more browsers support this change, you should continue to set fill. (Firefox bug 1973203).

• 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)
• Adds the browserAction.onUserSettingsChanged and action.onUserSettingsChanged events that listen for changes in the user-specified settings that affect an extension's action. (Firefox bug 1828220)

• 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).

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.