Probably your privacy settings or the long time session restore issue. What do you have Firefox set to do when closed. see screenshots

I have History set to Remember history.

I would also recommend running Firefox form Mozilla so that you can have your data in more than just one browser. I stopped running just my distro version about 6yrs ago when I first experienced the session restore issue. https://www.mozilla.org/en-US/firefox/all/desktop-release/linux64/en-US/ The versions are all run from my downloads folder. Just download, unzip, and run firefox-bin.

https://support.mozilla.org/en-US/questions/1509748#answer-1752029

I have History set to Remember history.

What do you have set when the browser closes? see screenshot

But if I have History set to Remember, then those options never come into effect, right?

Uncheck the 1st option in your screenshot, open a few windows with tabs and see what happens. Mine is not checked.

That screenshot from me was the wrong dialog. I don't have access to the dialog you are talking about unless I change the History option from 'remember' to 'custom' and then check 'clear history when firefox closes' - but unchecking the option made no difference. I have solved it however simply by running 'apt purge firefox' and removing the .mozilla/firefox directory.

The fresh install now re-opens tabs again. With a bit of luck this issue was just random and won't recur - otherwise I'll be looking at other work-arounds for the long-time session restore issue you mentioned, like using the Mozilla packages.

Great. Mark it as resolved and have a nice day.