Earlier quoted context omitted.
If UI changes needed everyone's approval, we would still be looking like a 1990's browser.
True, but if you're running a beta test and the feedback is negative, you can't be surprised that people don't feel "respected" when you decide to push the change anyway.
Overall FF is just as usable as the decade before in my opinion. And I kind of like the new tab design. The old super rounded tabs looked great for a while but it was time for a change.
I'm not a designer, but in my experience it helps to change up user interfaces now and then. However I wish we as users had more control over that. Color themes are not enough.