Firefox fell behind Chrome because of aggressive marketing from Google in a way that probably violates some antitrust laws if they were actually being enforced, combined with a couple of own-goals from Mozilla.

Basically Google exploits their market dominance in Search and Mail to get people to use Chrome (and probably their other services too). When you search in a non-Chrome browser, you'll be constantly informed by Google about how much better their search is with Chrome through pop-ups and in-page notifications (not browser notifs). If you click a link in the Gmail app on iOS, rather than opening the browser, you get a Chrome advertisement if they detect it isn't your default browser.

This goes hand-in-hand with Chrome being the default Android browser (don't underestimate the power of being the default) and Mozilla alienating their core audience of power users by forcibly inserting features that those power users despise.

Chrome never won on features, it won on marketing and abuse of a different monopoly.