Nothing odd showed up in my extensions, so I tried to just unistall, then reinstall Firefox. No joy. It was time to nuke Firefox from my system. Whereis Firefox and whereis Mozilla gave me my targets (I'm working in Linux Mint 18.2).
The offending whatever was in the .Mozilla folder. I reinstalled FF, and the heavens opened up and brought forth my tabs again.
Oddly, when I searched the net for info on the missing tabs I found several links to discussions about the tabs being permanantly removed as a new feature.
Defend your right to arm bears!