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There does not seem to be a way to add new a new address to the drop-down. I just get my usual address. Not much new with that.

هذا التعليق كان على إصدارة سابقة من هذه الإضافة (3.2) 

Identity Chooser simply shows all addresses that you've setup in Thunderbird.

If you add a new account (or a new address to an existing account), Identity Chooser will pick it up automatically.