Firefox 151 release notes for developers (Beta)
This article provides information about the changes in Firefox 151 that affect developers. Firefox 151 is the current Beta version of Firefox and ships on May 19, 2026.
Note: The release notes for this Firefox version are still a work in progress.
Changes for web developers
APIs
- The
Sanitizer.replaceElementWithChildren()method now returnsfalseif the element to be replaced is<svg>in the SVG namespace or<math>in the MathML namespace (joining<html>, which was disallowed in Firefox 150). (Firefox bug 2032359). - The Document Picture-in-Picture API is now supported on desktop platforms. This makes it possible to open an always-on-top window that can be populated with arbitrary HTML content. It can be used to display any content that a user might want to view separate from the launching page (or even the browser), such as a set of streams showing the participants of a video conference call, a stock ticker, or a countdown timer. (Firefox bug 2006594).
Changes for add-on developers
tabs.group()andtabs.ungroup()now correctly add and remove a split view when a call includes one of the split view's tabs. Previously, a call would fail or separate the split view. (Firefox bug 2029099)tabs.move()now correctly moves a split view to the right when a call includes one of the split view's tabs. Previously, a call moved a split view only to the left or to the end of the tab list. (Firefox bug 2027855)
Experimental web features
These features are shipping in Firefox 151 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.