Firefox 159 release notes for developers (Beta)

Changes for web developers

CSS

Removals

• The legacy masonry layout syntax has been removed: the masonry value of the grid-template-columns and grid-template-rows properties, and the masonry-auto-flow property. This syntax was only enabled by default in Nightly, behind the layout.css.grid-template-masonry-value.enabled preference. It's superseded by grid lanes layout, which uses the grid-lanes and inline-grid-lanes values of the display property. (Firefox bug 2077170).

APIs

• Methods that sanitize HTML with the HTML Sanitizer API, such as Element.setHTML(), now remove unwanted elements and attributes as the markup is parsed, instead of parsing all of the markup first and then cleaning up resulting DOM tree afterwards. The result is the same, except that neighboring text now lands in a single text node instead of being split across several. (Firefox bug 2070498).

DOM

• Scoped custom element registries are now supported, this allows a shadow tree to create an independent CustomElementRegistry whose definitions only apply to that specific DOM subtree. This can be used to avoid collisions where multiple web components declare elements with the same name. The customElementRegistry property can be used on Document, Element, and ShadowRoot. The CustomElementRegistry() constructor creates a new CustomElementRegistry object for scoped usage. (Firefox bug 2070506)

Changes for add-on developers

Experimental web features

These features are shipping in Firefox 159 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.

• CSS numeric factory functions exposed in workers and worklets: layout.css.namespace.exposed-in-workers-and-worklets

  The numeric factory functions of the CSS namespace, such as CSS.px(), are now exposed in workers and paint worklets, not only on Window. They're part of the CSS Typed Object Model API, so outside Nightly they also need the layout.css.typed-om.enabled preference set to true. (Firefox bug 2079188).

• New MathMLAnchorElement properties: mathml.a.element.enabled

  The MathMLAnchorElement interface now supports the download, ping, rel, relList, and referrerPolicy properties. (Firefox bug 2064612).