Make sure that you give Firefox enough time to close properly and save the session state to disk.
If you close Firefox immediately after closing a lot of tabs then try to wait 15 seconds before closing Firefox to see if that helps.

You can also check for problems with the file sessionstore.js.
You can check the date and time of that file to see if Firefox has saved it.

It is possible that there is a problem with the files sessionstore.js and sessionstore.bak in the Firefox Profile Folder.

Delete the files sessionstore.js [2] and sessionstore.bak in the Firefox Profile Folder.

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

If you see files sessionstore-##.js with a number in the left part of the name like sessionstore-1.js then delete those as well.
Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost, so you will have to create them again (make a note or bookmark them).

See also:

• [1] http://kb.mozillazine.org/Session_Restore
• [2] http://kb.mozillazine.org/sessionstore.js

I'm actually using IceWeasel 5.0 (Debian) which I didn't think was different. Deleting all sessionstore.* (there are only 2 on the whole machine) doesn't help. Waiting 60 sec. doesn't help. "Show my windows and tabs from last time" is not a choice under preferences.

Thanks,

Paul