That really does sound like a sync/profile issue, especially as it coincided with a failure to restore your previous session. I suspect you encountered some sor

That really does sound like a sync/profile issue, especially as it coincided with a failure to restore your previous session. I suspect you encountered some sort of corruption in your profile, and Firefox automatically restored one of the multiple backup copies it keeps to attempt to mitigate data loss. Still, always good to take precautions. What Firefox release channel(s) are you using? Are you running the same version across all of your sync'd devices? And can you share what change you made before restarting the browser?