These are the steps you need to read through for something like this:

https://support.mozilla.org/en-US/kb/how-do-i-restore-my-tabs-last-time#:~:text=Restore%20the%20previous%20session%20from%20the%20Firefox%20menu,-Here's%20how%20to&text=Click%20History%2C%20then%20Select%20Restore%20Previous%20Session.

https://support.mozilla.org/en-US/kb/restore-previous-session

https://support.mozilla.org/en-US/kb/how-restore-browsing-session-backup

https://support.mozilla.org/en-US/questions/1204253

If these don't then they are gone. Also whatever the last Browser of Firefox window is closed that is the session that is saved that can be restored from previous session options only. Otherwise you have to look at your history to find those missing sessions again.

Sorry, but yes, "Close All Windows" will only save the last 3 by default. There is a way to increase that for the future; I'll mention it below the line.

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future. Since we have had this setting for a long time and I use it myself, I feel comfortable mentioning it.

(2) In the search box in the page, type or paste browser.sessionstore.max_windows_undo and pause while the list is filtered

(3) Double-click the preference to display an editing field, and change the value to 6 (or whatever you think will cover you), then press Enter or click the blue check mark button to save the change.

Regarding recovering the upgrade files from sessionstore-backups, why do you say "it seems like the "upgrade" files are too large for it to handle"? Do you mean my Scrounger tool (https://www.jeffersonscher.com/ffu/scrounger.html) or Firefox itself?

Regarding recovering the upgrade files from sessionstore-backups, why do you say "it seems like the "upgrade" files are too large for it to handle"? Do you mean my Scrounger tool (https://www.jeffersonscher.com/ffu/scrounger.html) or Firefox itself?

I've tried to use the Scrouger tool and gave it a couple minutes. I've also set the number of windows to 20 now. I'm not sure why the default is 3 and not some unreasonable number most people won't hit, but this behavior is still very frustrating. I regularly use 3 windows, as I use multiple screens and like to multitask sometimes. Firefox is great for this, because it doesn't hog all my performance. But then I decide I want to do something else than browsing the web and accidentally lose the "spine" of my browsing behavior, because I used my OS like it was designed to be used.

That said, I have "solved" the issue by loading one of the backups in the folder, the most recent one being 3 days old. So anything I opened since then is gone and it's not in my history despite using Firefox Sync.