The design people in Firefox lost me as a user when they decided to re-arrange an interface that had looked the same for the better part of a decade. Re-arrange, not improve. Apparently years of users' visual + muscle memory means nothing to these UX guys.
I persevered with it for a while but eventually I just gave up and switched browsers. Shame, since I'd been using Firefox since its first release (called Phoenix back then, later Firebird, finally Firefox).
FWIW, here's the "original" user interface: http://upload.wikimedia.org/wikipedia/commons/6/65/Mozilla_F...