Like it or not, Apple and Google set trends in terms of look and feel.
This stuff is has all been proven, time and time again.
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.
The major improvements weren't cosmetics.
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.
This design feels very generic though. Doesn't really stand out from any other modern browser.