Rated 3 out of 5 stars
Great addon, however, breaks Firefox 4.X "Tabs on top" feature (not talking about the Tabs on top addon, this option is part of 4.X). It breaks it by adding a toolbar for the tabs that can not be moved
Author needs to update to follow Firefox 4.X tabs scheme.
Update: For those wanting to enable to "Tabs on top" feature found in Firefox 4.X, re-enable the option (View -> Toolbars -> Tabs on top) then open the TabGroups Manager Preferences in Add-ons, enter 20 in the "Ordinal of Tab Bar" field. This should move the tabs back above the URL field, if not, change the number until you find the right setting. Changing the number in "Ordinal of GroupBar" changes the location of the groups you created. From my testing a setting of 100 places this at the bottom of the browser window.