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

"cor-el - Delete the files sessionstore.js [2] and sessionstore.bak in the Firefox Profile Folder."

Thanks very much for your help. I found the folder and the files that I should delete. None of them were numbered. I have not deleted them yet because I do not understand the last part of your instructions (below). Can you clarify this any further for me?

"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). "

Thanks, Pete

Are you using App Tabs or Tab Groups?

• https://support.mozilla.com/kb/what-are-app-tabs
• https://support.mozilla.com/kb/what-are-tab-groups

The set up of those tabs is stored in the file sessionstore.js alongside with the regular session data, so removing that file will make you lose them.
You can make a note or bookmark those tabs if you need to restore them.