If you install the new version of the addon, it (should) pop up a little help message telling you how to hide the existing tab bar. Basically (as I detail in my

If you install the new version of the addon, it (should) pop up a little help message telling you how to hide the existing tab bar. Basically (as I detail in my other comment) you need to edit a file in the user profile directory. Sadly, the Mozilla team hasn't gotten around to adding a real setting for this yet (although there are plans to).