See if this helps. https://support.mozilla.org/en-US/questions/1449521

Thanks for the help, already tried all that and doesn't work. My scenario is similar, I could only restore the tabs where I was looking for solution (latest session), not those I had opened before this happened. Didn't even realise how it happened, I didn't experience a crash, the tabs just vanished all of a sudden, and no way to restore them even through the about:system hacks. Things should be easier, anyway. The browser should not just restore the latest session, but keep a record of the opened tabs in the last few days and hours.

You should post a screenshot of the files in your sessionrestore folder. There may be something there before the issue. You can see in my screenshot that there is a file from 10/12 that I could go back to by changing the name if needed. https://support.mozilla.org/en-US/questions/1440459#answer-1637875

Those files have been renamed and moved elsewhere, trying to solve the issue. It's not a big deal, thanks for the help, I can do without the lost tabs. My point is only that Firefox should have a proper session manager natively, without relying on extensions. I don't want to spend time with hacks, I've lost more than an hour today because of this.

Roger that. Mark it as resolved and have a nice day. Just FYI... I've been making copies of my sessionrestore folders maybe once a week or 2 because this does happen a wee bit too often.