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

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.

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

Hi Dany 00, did you back up the files from the sessionstore-backups folder? Since you just updated to Firefox 78 in recent days, the update snapshot might have most of your tabs. You can check it on my site here:

https://www.jeffersonscher.com/ffu/scrounger.html

As for the larger question of why Firefox might use an older file, I imagine it's because there was a problem with a newer file -- such as being locked or corrupted -- but that's just a guess.

Thank you so much for your replies, they were really helpful, ill be backing up the folder just in case, thanks.

My computer crashed (literally, I dropped it) and turned it off because I was getting no response. Then, every time I opened FF, all the tabs that had been opened at the crash reopened (even the Yahoo email that has long since been deleted).

I did the above and FF reopened in the one one tab I'd left open at closing.

OK! 

Then, I hit Home. The entire old session loaded.

My Home setting was that session. I updated, and all is well.

I may be the only one on the planet with this problem/answer. But, if not, here is the answer it took me a month to find.