Firefox 159 Enables Scroll-Driven Animations by Default and Adds Scoped Custom Element Registries; web.dev Lists Both as Baseline Newly Available
Firefox 159, released October 27, 2026, enables scroll-driven animations by default and adds scoped custom element registries, which web.dev lists as Baseline Newly available; legacy masonry syntax is removed.
Overview
Firefox 159 was released on October 27, 2026, according to MDN’s release notes for developers. The release enables scroll-driven animations by default and adds support for scoped custom element registries, both listed in the same notes. In its “New to the web platform in October” post, published October 29, 2026, web.dev lists both features as Baseline Newly available.
Scroll-driven animations
MDN says that “Scroll-driven animations are now enabled by default” in Firefox 159 and describes them as a way to animate property values along a scroll-based timeline rather than along the default time-based document timeline. The Firefox release notes say the change adds the animation-timeline, scroll-timeline, view-timeline and animation-range properties and the ScrollTimeline and ViewTimeline interfaces.
web.dev writes: “With support in Chrome, Safari, and now Firefox 159, scroll-driven animations are Baseline Newly available.” The post’s browser-support table for the feature lists Chrome 115, Edge 115, Firefox 159 and Safari 26. The WebKit blog’s Safari 26.0 feature post, dated Sep 15, 2025, names scroll-driven animations among the features arriving in that release.
In a DEV Community post dated Nov 4, author richi.codes says the demo checks CSS.supports('animation-timeline: scroll()') and falls back to a JavaScript scroll listener and an IntersectionObserver when the check fails. The author also reports that an overflow: hidden track made scroll(nearest) find the track instead of the panel, because hidden turns an element into a scroll container, and that switching to overflow: clip fixed it.
Scoped custom element registries
According to MDN, scoped custom element registries let a shadow tree create an independent CustomElementRegistry whose definitions only apply to that specific DOM subtree, which can avoid collisions where multiple web components declare elements with the same name. MDN adds that the customElementRegistry property can be used on Document, Element and ShadowRoot, and that the CustomElementRegistry() constructor creates a new registry object for scoped usage. The Herald’s earlier Firefox 156 coverage noted that, per MDN’s Firefox 156 notes, scoped custom element registries were then disabled by default and had to be enabled through about:config.
web.dev describes the problem this addresses: “Previously, all custom elements had to be registered in a single global window.customElements registry, which could cause naming collisions when combining components from different libraries or teams.” It adds: “With support across Chrome, Safari, and Firefox 159, scoped custom element registries are now Baseline Newly available.” Its support table lists Chrome 146, Edge 146, Firefox 159 and Safari 26.
A Chrome for Developers blog post published March 9, 2026, says scoped custom element registries are “now available by default from Edge and Chrome 146, as well as other Chromium-based browsers.” A DEV Community post by an author identified as JKC, dated Nov 7, says Safari 26.0 shipped the feature first and that Safari 26.4 added upgrading elements in initialize() plus fixes around null registries. The same author reports that document.createElement() still uses the global registry by default and that initialize() is first come, first served: after calling it twice on one shadow root in Chrome, the root kept the first registry.
Other changes in the release
MDN lists a removal alongside the additions: the legacy masonry layout syntax, meaning the masonry value of grid-template-columns and grid-template-rows and the masonry-auto-flow property. MDN says that syntax was only enabled by default in Nightly, behind a preference, and that it is superseded by grid lanes layout, which uses the grid-lanes and inline-grid-lanes values of the display property.
Among other developer-facing items, MDN says HTML Sanitizer API methods such as Element.setHTML() now remove unwanted elements and attributes as the markup is parsed rather than afterwards. The Firefox release notes add that the Web Console now allows let, const and class variables to be declared again so a snippet can be edited and re-run, and that, on compatible devices, WebGL supports the EXT_clip_control and EXT_texture_mirror_clamp_to_edge extensions while WebGL 2 supports EXT_texture_norm16.
What the sources do not say
The DEV Community post on scroll-driven animations notes that Baseline “newly available” means every engine ships a feature, not that every user has updated, and advises keeping a fallback for a while. None of the sources cited here reports adoption figures for either feature or for Firefox 159.