What's new in version 4.2?
Starting with version 4.2, Identity Chooser lets you enable or disable it on email drafts.
What's new in version 4?
Previous versions of Identity Chooser modified parts of Thunderbird's user interface directly by adding a menu with your identities/mail addresses to compose, reply and forward buttons. This is not possible anymore with MailExtensions. Instead, these buttons remain unchanged, and you can choose your sender address directly in the new message compose window -- immediately after the window opened.
This new workflow has the advantage that it works every time and everywhere Thunderbird opens a new message. This means, Identity Chooser doesn't only work in the main window; it also is active when you create a new email directly out of the address book, for instance.