I faced a strange behavior with a Firefox running on an Ubuntu notebook today: No access to the homepage, bookmarks not manageable anymore, no back nor forward buttons (all greyed). Last release of Firefox on top of an up-to-date Ubuntu.
After a few hours of investigations, the machine could be declared as “safe”: no suspicious traffic, nothing reported suspicious by Rootkit Hunter. Good point. The browser was not hijacked!
Starting from a clean config (mv $HOME/.mozilla $HOME/.mozilla.old) worked. Binaries and plugins were also clean.
Finally, I found a post on the Mozilla support forum (in Finnish!!!) the explanation: the file places.sqlite was corrupted! Once the file was renamed (to something like place.sqlite.old), Firefox worked as expected. No the $1M question is: how the file became corrupted!? (The laptop did not rebooted improperly for a while)
More information about this problem: Lock or damaged places.sqlite.
Sounds like something went tits up in an existing browsing session. But i would check your harddisk!.