What version of Firefox? I just had the same issue about 1AM this morning. Nightly did not restore my session, so I went the to sessionstore-backups folder to get one of the files to replace my sessionstore file, but there was none. No files in the folder. OMG right, but it turns out that Nightly had created another profile for some unknown reason. I discovered it by going to my profile folder and then going up 1 folder to the parent folder. There I saw several Nightly profile folders. see screenshot I looked in the previous folder and found the session files and was able to fix my session.

So, in your current browser, find your profile folder, go up to the parent folder and look for other profile folders. Post a screenshot of your sessionstore-backups folder.

Thanks for responding. I checked and I have only one profile. My incident actually happened Thursday. I needed to take a break from head banging and loaded Brave as my browser for now. But I really would like access to my old tabs. I found what I believe is the correct session right after I realized I lost my tabs on Thursday. Copied it to a safe spot. Also have older sessions copied to safe places. My problem is putting the copy in the profile directory is having no affect when I open Firefox. It just opens an empty browser or my last session if I did anything like download Brave. Opening Firefox is ignoring my sessionstore.jsonlz4.

Firefox 124.0.2

I assume Firefox is closed when you rename one of the files from the sessionstore.jsonlz4 file to the profile directory. If that file is not working, rename another file and try again.

All your session backups are large (3MB) so it must be storing something in there. Close Firefox, delete sessionstore.jsonlz4 from the root profile folder so it loads a backup and then open recently closed windows.