A Stack Overflow question led me to believe that our use of "keydown" is the problem, but this doesn't appear to be the case. What output appears in the Firebug console when you type enter into the textarea on the jsFiddle?
Title editing still not working, though. Firefox's debug console isn't saying anything either, which is very strange. It could be a problem with one of my extensions, I'll try disabling them and then post the results here.
There's another (possibly related) bitbucket compatibility issue on their issue tracker, I'll confirm/add to that. Sorry to waste your time with this, looks like it's probably not a problem with bitbucket at all :S