The recently launched Google Browser Sync seems to have lot of technical issues, and most important of all that the extension is enable to synchronize data at all. First the extension force to disable the SessionSaver extension so it become the only backup tool working on the current session, and then everytime I try to connect and save my session the synchronization fail.
Many people have reported the same issue, which could happen also during the first setup. After first trial I used to uninstall it and activate again my SessionSaver. at least after restarting firefox I know where I have stopped last time.
Recommandation : If you want to try the new extension Google Browser Sync beta, be sure to backup your work before loosing your session information.