I just had this problem after a toolbar install wiped my previous session of several tabs over several tab groups, about:sessionrestore was empty. I'm a little late to the party, but for the sake of posterity, here's how I got my session back.
I followed Xircal's first link, and decided to locate the firefox app data in window's users folder. Once I told explorer to show hidden files, I found the sessionstore.js and sessionstore.bak files I was seeking, as well as session.rdf, under AppData/Roaming/Mozilla/Firefox/Profile... I have Windows 7, but this should work if you have system restore active and backed-up regularly. I don't know if XP/Vista have the option in the context menu, or if those users would have to try a system restore.
Locate your firefox profile's AppData (above.)
Right-click to select "Restore previous sessions" on each sessionstore file, and session.rdf for good measure.
In the dialog that comes up, you have to click the restore button (there was only one previous version of each file for me.)
Launch firefox again and it should be back to normal.