I used system Restore to roll back Windows to 8/28/2014 to try to solve another problem (mouse freezes) and finally solved the problem with a different mouse, which installed a different driver. Problem coudl have been the mouse, the driver or both. But Firefox had lost all of its tabs.
I then rolled windows forward to the latest update, Firefox only gave me the "Well, this is embarrassing" dialog box,, and I was able to restore the tabs.
I think that the problem is that a Firefox update was making Firefox incompatible with its older sessionstore file, or something like that. Anyway, it works now.