Next time you get that sinking feeling that you've lost all your Firefox tabs, don't despair; there is a solution.
I'm a big fan of Firefox's tab groups; it really is sickening how many tabs I have "open" at one time. I admit I'm a tab hoarder, and that I probably need help.
To add insult to injury, I rarely back up the list of tabs I have open, so when I click on a URL in another program when Firefox isn't open, a new session is started from a clean slate. This means it appears that my tens of tabs have disappeared into the ether.
Enter that sinking feeling of information loss, and yet another vow that next time, it'll be different.
Fortunately, though, there is a way to restore the previous session.
Residing inside the .mozilla/firefox/[profile]/ directory will be a number of "sessionstore" files. This is a JSON store of the tabs that open in previous sessions.
When you have had a number of tabs open, and the browser opens without restoring the session tabs, a new sessionstore.js file is created and the previous file is renamed sessionstore-1.js.
To get all your old tabs back, all you need to do is close down the browser and replace the sessionstore.js file with sessionstore-1.js.
And voila, your old session should appear again. Now, if only I could cure my information hoarding and convince myself to close some of these tab groups.
Before any questions are asked about performance with that many tabs open, Firefox is clever about tab groups and will only load a group once you change into it, saving quite a bit of memory.