It's possible the file is corrupt. Have you tried any other files?

http://kb.mozillazine.org/Unable_to_process_the_backup_file_-_Firefox

jscher2000 has a tool on his website to extract out your bookmarks and save them to an HTML-format bookmark export file. If you want to try it, here is the page:

https://www.jeffersonscher.com/res/bookbackreader.html

To open the bookmarkbackups folder in Finder, use the "Open in Finder" button in the first table on the Troubleshooting Information page, and double-click into bookmarkbackups where you should see several dated files. From there you can drag and drop onto the conversion tool.

To import the HTML file, see the steps in this article: Import Bookmarks from an HTML file.

I have already converted a number of files through this site. Unfortunately, they contain folders with bookmarks. But I need a session of 400+ tabs.

Oh, you were talking about sessions. Sorry.

You may have corrupt sessionstore [v56] sessionstore.jsonlz4 file(s). Delete all sessionstore* files and the sessionstore-backups folder.

Don't delete the files if you need to rescue any data from them, just move them out of the profile folder to some location where Firefox doesn't look for them. You can try to read out their contents using this tool: https://www.jeffersonscher.com/res/scrounger.html

Type about:support<enter> in the address bar.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder

Linux: Under the page logo on the left side, you will see Application Basics. Under this find Profile Directory. To its right press the button Open Directory.

Locate the above file. Then rename or delete it. Restart Firefox.

Thanks to this tool: https://www.jeffersonscher.com/res/scrounger.html I was able to find closed tabs, but unfortunately deleting session files didn't help. I also tried converting to json for later opening in the log, but it also didn't give anything.

But you were able to recover what you wanted? Delete/move the sessionstore files. Do you still have the problem?

Yes and no. Okay. Thanks for that, too. Now at least there are tab links that I've lost.