The question is rather if Mozilla can win this catch-up game and win new users. If they do this they should rather IMHO simply upstream the stuff done by Zen [0], which IMHO has a much more modern look and feel. Btw: there is still quite some users that are lost if UI changes. Particularly older people might simply use pre-installed Edge if they have to get used to a new design anyway. My parent in law were still on SeaMonkey until a few months. I installed some weird old school theme (I think echolon [1]) to ease the transition.
I do not redesign my (physical) toolbox multiple times per decade.
On the contrary: I use my grandfather's drill, because it lasts longer and gets the job done more reliably then the "redesigned" crap that is most of the current market.
Do I still use FVWM2? Hell no...
Could I work the same like 30 years ago, if all the fashion wouldn't have happened: absolutely.
Re-litigating assertions is sometimes incredibly useful.
Firefox isn't in the spot it's in because it doesn't copy Material Poo/Liquid Ass enough, or because it doesn't look like Chrome enough, or that in a somewhat crowded market just doing your best to blend in with the rest is a good strategy for growth or popularity. Refusing to follow fashions that make your browser harder to use or uglier might not be a terrible idea. Mind you, I'm not trying to claim that it absolutely is a great idea, or that Mozilla should copy the Winamp skin and go backwards, but goddamn, have some guts.
But I guess it's an axiom, that if you don't copy how everyone else looks, you're doomed.
Like it or not, Apple and Google set trends in terms of look and feel.