View source for Features/Thunderbird/Address Book in a Tab

The justification for this change is a consistent UX. Thunderbird has been moving towards new tabs over new windows for a while now. The following interfaces open up in new tabs (in no particular order): 

#The Add-ons Manager 
#The What's New page (Help &gt; What's New) 
#Troubleshooting information (Help &gt; Troubleshooting information) 
#Messages, when double-clicked in the message tree 
#Lightning

There are likely more. 

As for the interfaces that open up new, stand-alone windows, I can only think of the following: 

#The compose window 
#The address book 
#The activity manager

If the Compose-in-a-tab experiment is any indication, Thunderbird will be moving away from new windows entirely (or, rather, new windows by default - note that tabs can be "torn" out to form a new window). 

If that's the case, then the address book will be the lone holdout, and this could be very confusing for a user. We have spent a lot of energy training users that new things appear in tabs - but only one or two things open up in new windows. That's unnecessary cognitive load. 

Also, from an add-on development perspective, there are certain disadvantages to having the address book open up in a new window. The primary one is that once the address book window is open, we cannot guarantee that the 3 pane (main Thunderbird window) is open, which means that we cannot guarantee access to a lot of the useful functions available exclusively in the 3 pane window. Having the address book open up in a tab within the 3 pane would eliminate this edge case.