In Firefox 9, the Tools->WebDeveloper->Error Console had the following error:
session file is invalid type error this_initialState.window is undefined
This occurred after FF crashed and I had to kill it - even reboot - and such as it the whole system was bogged down by something. I couldn't find anything on-line about it and started digging in. Firefox would not restore my session - all the tabs and tab groups, etc.
I did a comparison with the new sessionstore.js file that it had, and found that there was a little section that was missing from the default, but present in the backup - in bold below:
I removed the text, copied the resulting sessionstore file over sessionstore.js, and viola - firefox reloaded everything!
Hopefully others that have this issue will find this post and not have to spend a couple hours trying to figure out how to get their data back.