Scroll-driven animations are now enabled by default, animating properties along a scroll-based timeline. Scoped custom element registries let each shadow tree have its own CustomElementRegistry, avoiding name collisions between web components. The legacy masonry syntax is gone in favor of display: grid-lanes. Plus, setHTML() now sanitizes during parsing and WebGL gets new extensions.
developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/159