Implemented Intl.PluralRules API for JavaScript.
Added a frequently-requested preference (browser.tabs.allowTabDetach) to disable "tearing off" of tabs (meaning dragging them outside of the tab bar resulting in them being made into their own window).
Added FLAC as a recognized filetype-by-extension.
Implemented basic support for the scrollbar-width CSS keyword. See implementation notes.
Added preliminary support for modern FreeBSD builds.
Selectively enabled core features of the DOM Animations API.
Enabled AV1 video support by default (previously built but not enabled in releases).
Added support for pointer events.
Added support for the SVG transform-box property.
Added support for the inputmode property for forms to enable context-sensitive display of soft keyboards.
Enabled shutting down of the file I/O worker when idle for a while (resource optimization).
Enabled blocking of auto-play of media in the background by default.
We now offer official GTK3 builds for Linux alongside the GTK2 builds.
Partial (and as of yet, not acceptably functional) implementation of Google WebComponents. See implementation notes.