I think you vastly overestimate "everyone else". I use Firefox and didn't even notice the change. So tabs don't have borders now, ok honestly whatever... Maybe try not to get hung up on these things and move on with life.

Same. I think I've just gotten used to things changing a little bit over time.

Releases are generally well thought out enough that they don't break any mental models or workflows, even when the UI noticeably changes (which in this case, it didn't for me)

UI people seem to start with the idea that they're right and the user should just suck it up, because they went to school and learned words like "skeumorphic." This is apparently sufficient for middle management, despite the fact that when this trend of constant annoying pointless changes started, so did the decline of Firefox's user base.

I don't even install it by rote on new computers anymore - it might as well be Edge. The rendering performance sucks, the need to constantly adapt to new ui changes is infuriating, so buh-bye Firefox. I prefer software that doesn't screw with my muscle memory for absolutely zero good reasons.

UI as a "discipline" is cargo cult corporate waste. I'd rather use a tool designed by an engineer that uses it than performance art disguised as ergonomics or accessibility.

If it looks good, fine, but the point of a browser isn't pretty tabs, it's navigating content in a way that doesn't interfere with navigating content.

I do not. I’m a software engineer. I prefer clean and minimal interfaces and I think Firefox does a good job with that. And judging by some other responses, I’m happy to hear that I’m not alone.

I like functional interfaces, where you can distinguish clickable fields from each other. And where you can distinguish clickable from non-clickable fields. And judging by some other responses, I’m happy to hear that I’m not alone.