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
HTML
- The
shadowrootslotassignmentattribute is now supported on<template>elements, allowing declarative definition of slot assignment behavior for shadow roots. The attribute is reflected in JavaScript byShadowRoot.slotAssignmentorHTMLTemplateElement.shadowRootSlotAssignment(Firefox bug 2031295, Firefox bug 2023824).
CSS
-
The
@containerCSS at-rule supportsstyle()queries. This allows you to check if a container has a valid CSS declaration, a CSS property, or a custom property, and apply styles to its children accordingly. (Firefox bug 2016929, (Firefox bug 2019254), (Firefox bug 2024601), Firefox bug 2030645). -
The
position-anchorCSS property now supports thenormal(default) value. This allows the matching of theposition-areaproperty, so if theposition-areaproperty updates theposition-anchorproperty will also. (Firefox bug 2030351).
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).
- The
CanvasRenderingContext2D.langproperty is supported for setting the language of the canvas drawing context. While a DOM canvas can inherit this context from thelangattribute of its associated<canvas>element, this is useful for setting the context for an offscreen canvas, which may be rendered before being associated with a<canvas>. (Firefox bug 1943070). - The
options.keyboardLockproperty can now be passed as an option toElement.requestFullscreen(), allowing websites to request keyboard lock when the element is displayed in fullscreen mode. This stops the Esc key from causing the element to leave fullscreen (a long-press is required instead), and some formerly-reserved browser hotkeys can now be intercepted and their default action prevented. (Firefox bug 2032302).
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.
-
@container style()range syntax queries:layout.css.attr.enabledThe
@containerCSS at-rulestyle()queries now supports the range syntax. This allows you to check if a container has a valid CSS custom property and compare its value with comparison operators like>,<,>=, and<=and apply styles to its children accordingly. (Firefox bug 2024601).