I remember when it used to look like a nice, compact native dialog box* instead of a full-screen, space-wasting abomination that follows no standards for anything.
But consistency, visual coherency, discoverability and predictability are apparently all outmoded concepts in UI design, so opinions like mine -- that native applications should look and feel like native applications -- don't seem to be worth much among the cool crowd nowadays.
I disagree with you on this particular point, but in a wider scope I think you're spot on - it seems so little thought is given to the very basics of UI design.
I've just opened preferences and I don't see much improvement. It's a page now, and there's less stuff, but it's what I had 15 years ago. Anyway, I still have to go to about:preferences all the time.
I totally dislike their experiments with menu bar. I never wanted that. It's Ribbon all over again.
I might have said that Preferences was much the same as ever, for most practical purposes, with only comparatively minor improvements; but one thing particularly stands out as a massive improvement: the search box. Now I don’t need to know where what I want to tweak is, if only I know what to search for, which I pretty much always do.
I didn't find any UI improvements in the roadmap, other than password manager. I also remember that password manager was added to Opera browser around 2000. It wasn't a great sucess since nobody figured out how that's different from regular "remember password".
A number of innovative browsers never gained traction, and when mainstream ones try to improve UI it gets considerably worse.
Every new browser has a short runway of new awesome ideas and then it's stagnant for decades.