It might just be the nature of computers. A similar thing happened to me on Chrome during a recent update. I was using it as a backup browser and lost everything there when Chrome stopped working. I deleted it from my laptop and haven't missed it at all. I now use multiple Firefox profiles to work as alternate browsers.
If you go into your profiles directory is the old profile still there? It might have just created a new profile for you.
Chrome did it to me too. I had many links saved in onetab that are now lost. No software is completely stable on all computers in all situations.
Did you double-check your profile folder? Firefox might have created a new profile for you while leaving the old profile there. If you're on Linux or Mac, try typing this in a terminal:
firefox -ProfileManager -no-remote &
Then, if there is more than one profile listed there, try each one to see if it's the old profile.
(I'm not sure how to find the profile folder on Windows.)
If you go into your profiles directory is the old profile still there? It might have just created a new profile for you.