Why do they need to change? The tab bar has been perfectly functional for decades, and the relatively recent change makes it worse in many aspects.
I don't mind changes that fix problems or introduce new useful features, but don't fix what ain't broke. And no, some designer/product manager somewhere having to justify their salary doesn't count as a valid problem in my book.
Also, your comment about designers and product managers is pretty dismissive and leads me to believe that you aren't very appreciative how hard it is to get UI/UX right. Just because it's area you don't understand or don't practice within doesn't mean it's not important.
I don't recall every seeing anyone complain about the existing UI though (either from FF or other browsers implementing similar tab bars), so seems like most "others" were also perfectly satisfied with the existing UI. A browser tab bar is a solved problem and attempts to mess with it usually end up worse, both for Firefox and even for Safari's as their recent-ish attempt demonstrates - which they ended up thankfully rolling back in a subsequent update (the "separate" tab display option now behaves like it used to but there was a time when even the separate option added a disgusting amount of whitespace, along with a bug that would crash the browser if you dragged a tab while it was loading).
> even though the changes are for a product and user base as a whole
You mean the single-digit marketshare which is mostly power users, a good chunk of whom is here on HN always complaining about their changes because they're so out of touch with their current users? Or the user base that doesn't exist and never will because those are perfectly happy with Chrome and don't want to switch to what looks to them like a slower, knockoff version?
> you aren't very appreciative how hard it is to get UI/UX right
I absolutely am aware how hard it is to get those things right. I also believe that once you do get it right (through a decade of effort, experience and feedback) you don't ruin it on a whim like Mozilla did. Today's low-effort trend of replacing everything with whitespace and removing most affordances is also a major step backwards and this isn't limited to Firefox.