Hi Tom,
It won't be easy to spot the exact cause of the behaviour, but I can already give a single advice at this moment: Don't use FF in the admin backend for v3.x
As from v4, FF and IE will be fully supported.
Hope that helps.
Regards,
/Dirk
level 1 & 2 certified - umbraco MVP 2008/2009 - umbraco blog at
netaddicts.be - working on an integrated forum4umbraco