This release expands attr() to any CSS property with unit identifiers and fallback values, adds the progress() and alpha() functions, renames font-stretch to font-width, and brings Promise.allKeyed() and Promise.allSettledKeyed(). DevTools get a media feature emulation panel and JSON Lines support. Behind flags: scroll-driven animations, scoped custom element registries, and line-clamp improvements.
developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/155