The release adds shadowrootslotassignment on <template> elements for declarative slot assignment, @container style() queries, position-anchor: normal, the Document Picture-in-Picture API for always-on-top HTML windows, desktop Web Serial API, CanvasRenderingContext2D.lang, keyboardLock in requestFullscreen(), and the experimental field-sizing property for form controls.
developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/151