To move tabs and tab groups from one computer to another (or, as was the case for me, from one op sys on one partition to another), do NOT use sync.
Firefox sync does NOT move tabs and tab groups.
I spent two hours setting up Firefox Sync specifically to move 142 tabs from one Firefox implementation to another. Then 30 minutes trying to find out why it did not work. Answer: because it doesn't do it.
Instead, move the file sessionstore.js from one to the other: this file contains all the tabs and tab groups.
sudo updatedb locate sessionstore mv /home/simon/.mozilla/firefox/blah1234.default.sessionstore.js /home/simon/.mozilla/firefox/blah1234.default.sessionstore.was cp /<other place>/sessionstore.js /home/simon/.mozilla/firefox/blah1234.default.sessionstore.js
Restore Previous Session is greyed out and the previous session was not restored is a problem since Firefox 25.
Edit sessionstore.js and remove from near the start:
so a file that starts with
becomes, with the removal of the 39 characters ],"selectedWindow":0,"_closedWindows":[