Firefox 156 release notes for developers (Beta)

Changes for web developers

WebDriver conformance (WebDriver BiDi, Marionette)

General

• Marionette and RemoteAgent now both use a custom exit code (69) when their server fails to start. (Firefox bug 2040974).
• Improved the timing of intermediary events for actions with a duration greater than 0, to be closer to a 16ms interval and avoid inflating the overall duration even if the content process is overloaded. (Firefox bug 2054442).

WebDriver BiDi

• browsingContext.startScreencast will now safely pick a valid download folder and should no longer throw if the default download folder (DfltDwnld) is not available. (Firefox bug 2066782).

Marionette

• The WebDriver:GetElementTagName command was updated to match the latest specification changes and now returns the DOM element's qualified name. This command used to always lowercase the return value. In practice, this change is backward compatible for HTML elements, but it is a non-backward-compatible change for elements with a case-sensitive qualified name, such as SVG elements.(Firefox bug 2026697).

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.