I have upgraded to 12.04. I have noticed that Firefox Sync does not work again. When I have tried to set it up again, I came into the pop-up window, filled in all the required fields (I am using the recovery key) but when pressing Next nothing happens.
Are you still encountering this issue? If yes, I suggest you either:
Latest version of Firefox is now 16.0.2 on Ubuntu 12.04 LTS.
Hope this helps!