Firefox 156 release notes for developers (Beta)

Changes for web developers

CSS

• The non-standard ::-webkit-scrollbar pseudo-element is now reported as unsupported in @supports conditions on every site, so @supports selector(::-webkit-scrollbar) returns false and @supports not (selector(::-webkit-scrollbar)) returns true. This includes the sites listed in the layout.css.fake-webkit-scrollbar.enabled-domains preference, which Firefox 155 limited the pseudo-element to, and where Firefox does still apply it. Sites use this check as a signal for the whole ::-webkit-scrollbar-* family, of which Firefox implements only ::-webkit-scrollbar itself, so those that guard their standard scrollbar styles behind @supports not (selector(::-webkit-scrollbar)) now get those styles applied. (Firefox bug 2062782).

Changes for add-on developers

• The theme manifest key adds the backgrounds_area property. This property enables a theme to specify where its background images and gradients are drawn. Setting it to "window" draws them across the whole browser window, while "top_toolbars" restricts them to the horizontal toolbars at the top of the window. When backgrounds_area is omitted or set to "auto", Firefox chooses the area based on properties.additional_backgrounds_alignment. (Firefox bug 2059526)

Experimental web features

These features are shipping in Firefox 156 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.