Hey man!
My favorite FF add on is Undo Closed Tabs Button. Damned handy at home and at work.
You probably already found this, but All-In-One Sidebar is pretty handy when detoxing from Opera. As is DownThemAll!. PDF download is also handy.
I like FoxTabs for work, but rarely use it at home.
Really other than being (IMO) alot slower, the only thing I really miss anymore is the lack of the integrated ability to neatly save and restore multiple tab sessions.