I don't like it either, but with userChrome and https://github.com/MrOtherGuy/firefox-csshacks I can put it back to how I like it.

I'm dreading the day Mozilla removes userChrome though, and I'd be very tempted to start looking for an alternate browser at that point.

I cannot fathom why Mozilla won't leave, and promote, userChrome for the tinkerers and power users.

It could just sit there behind a "This may break stuff, only enable if you're OK with this" preference and they'd placate the users who want to customise Firefox and keep users who just want a working browser away from unwanted side effects.

But I can't see that happening, and the reasons Firefox became popular in the first place (new features, customisability and a expanding group of evangelising users) don't exist any more.