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Just about all of them have the H# tags garbled. On the recent changes page there are random stretches of text garbled. (Not really random, because it's the same places every time I visit.)Which pages on the Wiki are you having problems with? I just had a quick browse around in FF3 and everything looks fine...
It's not just MediaWiki sites either. I was browsing the CSS area of w3.org thinking to check out some of the new capabilities, and the H# tags are garbled there too. I don't know what's going on, but I may downgrade back to FF2 if I can't figure it out.