Do you want your livingroom furniture to be rearranged every six weeks in ways that make no sense, provide no benefit, and reduce comfort and familiarity for no other reason than 'it's what everyone else is doing"? On top of that the furniture gets nailed down to the floor so you are unable to change it without putting in some work and effort? :)
Now that rant begins. :)
Maybe I am crazy, perhaps slightly mad, but I detest and find incredibly irritating UI changes that provide no increase to the UX and most of FF UI changes are like this. A perfect example from a couple releases ago is when Firefox renamed "Undo close tab" to "Reopen Closed Tab." What is the point of that? None! What issue does it fix? None. What improvements does it bring? NONE! And yes, it's a simple 10 minute "fix", but that is besides the point.
I never dug into the reason why this was done, but I can imagine some VP proposed this change to show others what they are getting for her/his $600K salary, then sent it down to the Director, who told the PM, who told the Manager, who told the team leader, who told the senior developer,who told a junior developer to "fix this issue" and everyone along the way didn't speak out about the frivolousness and pettiness of that change, perhaps for fear of reprisal. Yes again,it is a minor change, but it is a perfect example of what Mozilla thinks is important and what Mozilla prioritizes. Fluff over function.
Rule #1 of UI design is if the UI works well then leave it alone. Don't add 50% more whitespace to context menus just because the other browser did it. Don't add round corners just because someone else did. Don't put grey text on gray backgrounds just because everyone else does it.
It's not just Mozilla, but UI change for the sake of change has permeated into all areas of software, but that is for another discussion at another time.
I still use Firefox as my main browser on desktops, but now only because it reliably disables autoplay of video & audio. If Vivaldi gets off their butts and implements autoplay disabling functionality, then I likely will switch to that.