Hi

I have looked into this and have this guidance from a more knowledgeable fellow contributor:

Firefox's tab session saving function is pretty robust, however there are special situations where a browsing session may be lost (often when the process gets killed while shutting down, a la Windows 10 update). Here's a primer on recovering from that.

Check about:sessionrestore

If a session is lost, hopefully the restore page opens automatically, but if not go to about:sessionrestore in the location bar. If this page is empty, keep reading.

Restoring from a backup file

• Go to about:support in your location bar and click on the Open Folder button in the Profile Folder line. A filesystem view of your profile folder will show up.
• Make a copy of the sessionstore-backups folder and shut down Firefox. [Generally you want to avoid filesystem operations (move, copy, delete) in this folder while Firefox is running. The copy of the unaccessed backups folder was necessary and should be safe.]
• Move the file called sessionstore.js to another location (like your desktop). [Note that this file does not exist while Firefox is running.] Moving rather than deleting is a precautionary step.
• Navigate to the sessionstore-backups backup folder you made.
• Review the files in this folder by date modified (you can ignore their names, any files at all in this folder should be restorable). If the most recently modified is particularly small in size compared to others, it may be mostly blank and you'll want to use the bigger one. (Don't stress on this, you can perform this procedure multiple times.)
• Copy your chosen file into the the main profile folder.
• Rename‡ the file to sessionstore.js
• Start up Firefox

If you feel that you are more prone to losing your session(s) with lots of tabs may fall into this category - or would just like insurance, the Session Manager addon does a good job at keeping sessions safe.

‡ Please note: You may need to ensure that your file system viewer is not hiding the file extensions of known types (like .js) in order to properly rename the backup files.

I hope this helps, but if not, please come back here and we can look into a different solution for you.

As a P.S. to the above, if your current sessionstore-backups folder doesn't have a usable recovery.js/previous.js, you can then look in the sessionstore-backups folder inside Old Firefox Data on your desktop.