Ho sospettato anche io potesse essere un problema di profili, infatti io uso un profilo che condivido in win, e per questo si trova su una partizione ntfs, ma anche utilizzando il profilo di default(su linux) non va:
lenny:~$ firefox -ProfileManager
No bp log location saved, using default.
[000:002] Browser XEmbed support present: 1
[000:002] Browser toolkit is Gtk2.
[000:002] Using Gtk2 toolkit
[000:009] Warning(clientchannel.cc:560): Unreadable or no port file. Could not initiate GoogleTalkPlugin connection
[000:009] Warning(clientchannel.cc:403): Could not initiate GoogleTalkPlugin connection
[000:009] Warning(optionsfile.cc:22): Load: Could not open file
[000:009] Warning(clientchannel.cc:530): Failed to get GoogleTalkPlugin path. Trying default.
[000:022] Started GoogleTalkPlugin, path=/opt/google/talkplugin/GoogleTalkPlugin
[loop][000:022] Waiting for GoogleTalkPlugin to start...
[001:105] Warning(clientchannel.cc:560): Unreadable or no port file. Could not initiate GoogleTalkPlugin connection
[001:106] Warning(clientchannel.cc:403): Could not initiate GoogleTalkPlugin connection[/loop]
Eppure FF, come detto, vede entrambe le estensioni: Google Talk Plugin e Google Talk Plugin Video Accelerator