Along those lines I just got screwed by FM and can no longer select themes. I've always used the "classic" theme to be able to see some specific headers plus some debug headers I add from my sieve script.
While I was looking at a message all the displayed headers disappeared and were replaced by "show details". I now discover I cannot select classic them because the general settings for Theme say "Sorry, your browser does not support this". So their screwing around with the UI is pi$$ing me off at the moment.
I use an older version of OS X and and older version of Firefox and I don't want to upgrade my OS for personal reasons. I'm guessing their screwing around that affected you is also affecting me. Or they just don't really care about older browser support.
I too submitted a ticket (before reading the latest posts in this thread).