Perform the suggestions mentioned in the following articles:

• Restore previous session - Configure when Firefox shows your most recent tabs and windows
• How do I restore my tabs from last time?

Check and tell if its working.

You can check if there is a numbered sessionstore-##.js file in the Firefox Profile Folder from the time that you still had those tabs.
If you find such a file then make a backup copy and rename the file to sessionstore.js to see if that brings back the lost tabs.

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Open Containing Folder
• http://kb.mozillazine.org/sessionstore.js

None of the older sessionstore-##.js files had the AppTabs, but thanks anyway!

There shouldn't be any sessionstore-##.js files with as number. If you have them then that means that Firefox wasn't able to rename the temp sessionstore file with the number to sessionstore.js
You can remove all those sessionstore-##.js temp file with a number.