Firefox 123 for developers

This article provides information about the changes in Firefox 123 that affect developers. Firefox 123 was released on February 20, 2024.

Changes for web developers

Developer Tools


  • The <template> element now supports a shadowrootmode attribute that allows declarative creation of a shadow DOM subtree. The attribute can be set to either open or closed, which expose or hide JavaScript in the shadow DOM from external code, respectively. These are the same values as the mode option of the attachShadow() method. (Firefox bug 1870052)


No notable changes.


  • The Date.parse() global object has had a number of bug fixes to bring it into line with how other browsers parse the values being passed.
    • Incorrect day of month (e.g. "31 April") now skips over to the following month (e.g. "1 May"). (Firefox bug 1872333).
    • Incomplete time zone (e.g. "1/1/70 gm") or AM/PM (e.g. "1/1/70 10:00 a") are no longer accepted. (Firefox bug 1870570).
    • Single number dates are now accepted (e.g. Date.parse("0") now returns 946684800000 - Sat Jan 01 2000 00:00:00). (Firefox bug 1870434).





Media, WebRTC, and Web Audio


The IDBLocaleAwareKeyRange interface has been removed (Firefox bug 1730706).

WebDriver conformance (WebDriver BiDi, Marionette)

WebDriver BiDi


Changes for add-on developers

  • Addition of the contextualIdentities.move function enables items to be moved in the list of contextual identities. This function enables extensions to customize the order in which contextual identities display in the UI (Firefox bug 1333395).

Experimental web features

These features are newly shipped in Firefox 123 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