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Although I only upgraded my Mac's browser to Firefox 3 this morning I will be downgrading it shortly
Several web applications I use on a regular basis break badly in Firefox 3, which renders them unusable (typo3) or not as usable as I would like (Movable Type).
I know the guys in Movable Type will fix the issues, though they were waiting on a stable Firefox release before they made any crazy changes.
I've no idea how long it will be before the Typo3 team fix the UI issues on their end.