I don't know. This happened to me once, and it was due to having another instance of Firefox open thanks to a link getting opened in a new window. Here's how it all went down: I forgot to close that new Window when I was done with it. Instead, I simply switched back to my main window with all my tabs (just as though I wasn't done with the new window), and I kept on going for a few more hours, and then I closed Firefox when I was done. When I did that, I was greeted by the old new window that was still open with just the one tab, so I closed it too without giving it a second thought. So to recap: I closed my main window that had all of my tabs, and then after that I closed that new window that got open thanks to a link opening itself in a new window. So the next time I opened Firefox, I was greeted by that one single tab. I lost all of my tabs because the last Firefox I had open only had that one tab in it.
From this experience, I can tell you that your only hope is going through your history to find the moment when you opened Firefox because all of those tabs opened all at the same time. Although, this can be hairy because each time you visit something that is in your history, it gets moved up. It might be the only way.