if Mozilla made the best browser ever, would that really make a difference?

I really don’t think it would.

One thing “Do what you did before when you were successful!” is missing is that Firefox was sleek, fast, extendable, new, and cool.

It was cool partly because of its “sleek, fast, extendable”, but also because of its “new”, and it can’t get “new” back.

Perhaps launching a completely new piece of software, like the Netscape -> Firefox change (although that was organic, not planned) - but only if it’s actually something new and cool that comes out of it. A rebadge won’t work.

I don’t think there’s a playbook that works here. I’m struggling to think of many major pieces of tech (or non-tech!) that ever got its cool back. Netscape -> Firefox? Apple and/or the Mac? IE4? Lego? Elvis? “New Nixon”?...

Alongside all of the successes there are orders of magnitude more failures, and I don’t think they’re all on merit.