Unable to reopen Tab Center

Archives Test Pilot/Tab Center
#1

I’m running Linux (Ubuntu 16.10) and Firefox 51.0.1 (64-bit. I moved the tabs back to the top and now I do not see a way to move them back to the side bar. Is there a way?

#2

There should be a tiny button that looks like this on the top right of the window…

#3

It’s there, but almost invisible!!

#4

Thanks for the help!

#5