Rated 5 out of 5 stars
An excellent little add-on that makes the address book behave as one would expect of a tabbed Thunderbird (this should be the default behaviour).
It would be even better if it used the default address book icon and/or placed the button in the mode toolbar (alongside the calendar and tasks buttons added by lightning).
One note. I have identified a minor incompatibility (more of a quirk really) between this add-on and the hide menubar add-on. Hovering over the main window menubar brings up a menubar for both the main window and the address book tab (it looks a little odd, although it could be useful).