However personally i do not like how these particular "tab buttons" look like and if nothing else (they remind me of those long pills that often feel hard to swallow :-P), i am used to them looking like tabs and see no reason for that change (fortunately Firefox allows you to customize its look and i have a userChrome.css that makes it look more to my liking).
Almost everything nowadays is designed with mobile-first in mind approach - whether it's a smartphone or tablet, or desktop software. That throw all semantics out of the window - look what happen to e.g. Gnome over the years. What's worse I'd say, is that the lack of clear differentiation between types of interface elements made easier to hide options within GUI under various dark patterns (active element vs static information etc.) - whenever its required to do so.
As for Firefox GUI changes: Mozilla ask their users for feedback many times and the feedback was given - often strongly criticizing the upcoming changes but they ignored it and introduced changes anyway. I did submitted mine when they were about to rollout Australis but I didn't bother myself to say anything when Proton was about to be introduced because I knew that the corporate facet of Mozilla doesn't care and they'll do whatever they like.
But I have a problem with a lot of the Firefox themes making it very difficult to quickly see which tab is the active one. I generally look for themes where this is obvious.
The overall state of UX is very sad for Firefox and also Thunderbird (I had to stop using it after their menu bar fiasco and other "improvements").
Thunderbird was ruined with version 115, so I switched to Kmail. I miss calendar integration in my email client though.
But no, I don't have a problem with Firefox's tab style. It's immedaitely learnable. I've never once second guessed which was the active tab or what those things up there are.
What are the odds of that person identifying what is currently active?
If the counter is "you know because you remembered" or you "know because you learned" then any of these answers indicate a inferior and non-intuitive UI design.