Hi,
it seems that the latest version of the add-on broke tab groups for me on Ubuntu 14.04 / FF 35.0.1. The menu item does not appear any more and when I click on the tab group button in the tool bar nothing happens, even though I have multiple tab groups.
When I remove the add-on, then a click on the tab groups button has the expected effect again. If I then install the add-on, I see the menu item - but after the next restart, the menu item has disappeared again and a click on the tab groups button again shows no effect at all.
Regards
Christoph