That's already happened, and they still haven't changed anything. Even Microsoft was wise enough to jump ship to Chromium, and arguably their last in-house Edge was a superior product to Firefox -- lean and mean and well-thought-out, but still failed because it wasn't Blink. Firefox doesn't even have that, but they stay the course because what, stubbornness? Ideological purity? Poor leadership? I dunno.
Firefox just isn't a meaningful contender on any front anymore. UX is slightly worse than other major browsers, privacy isn't significantly better than Safari or Brave, syncing is more complex than Apple/Google because the Mozilla ecosystem is browser-only, its ads are more intrusive because they're bundled as built-in adware, etc. It doesn't do anything better anymore. It's just riding its coattails from the 2000s, like Opera (which also jumped to Chromium).
If Mozilla wasn't a nonprofit sponsored by Google (oh, the irony), it would've been bought out by a no-name Chinese venture firm a long time ago.