Open Firefox browser today and none of the tabs open. How do I fix it?

What OS? If windows, what security software besides MS Defender are you running? What version of Firefox?

Yes. It happens so often that I've been making a copy of the sessionstore-backup folder. I update it when the current one gets old and before updates. I also have several in the trash.  ;-)) Can you post a screenshot of the files in your sessionstore-backup folder in your profile? see screenshots