They're always planning things and always last at things. Even for the most visible and painful cross-browser issues, they always last. One thing being service workers on extensions: available in Chrome since 2020 and we're closing 2024 still without them in Firefox.
Firefox is absolutely not always last at things. Chrome, Safari and Firefox are all often first, and all often last.
Chrome tends to ship things its team comes up with first, and things the rest come up with last. (This should not be considered surprising, for any team.) It tends to ship things half-baked (specification incomplete, or buggy implementation, though that latter isn’t as common as it was ten years ago), and often things both Safari and Firefox deem harmful. Firefox and Safari tend more to ship things only when they’re complete (specification and their implementation).
Especially in cases like this, Firefox was the first mover (<style scoped>), but then (simplifying the story slightly) Chrome nudged development in a different direction (because of Shadow DOM considerations, I think), and so Firefox eventually removed what they’d done and saying they’d wait until the dust had really settled before implementing again.
That's because nowadays the "standards" is "just what chrome does", and the other browsers can only catch up.
Witch is easy for edge, opera and the others chrome-clones, but not straightforward for the rest - aka ff.
as other as said this feature was already present in ff only, but then noone picked it up and so it was abandoned
