I was one of the guys with the problem, found a solution for myself. You need to initiate a new tab group REGARDLESS OF ALREADY HAVING TAB GROUPS SAVED IN A RESTORED SESSION. In my case, I uninstalled and reinstalled Firefox due to corruption and I had to recover my session with 107 tabs in about 8 tab groups. My settings were wiped, and the icon was missing for tab group switching. I remembered the tutorial on tab groups and realized that the icon creates itself after some sort of use of the tab group directory. When switching between groups didn't do the trick, I double-clicked on a blank spot to initiate a "new" tab group, and instantly the icon appeared again. Just deleted the blank window and switched back to the tab group I wanted to work with. Hope this works for you.