How does it work?
Whenever you open an email editor to write a new email, reply to a message in your inbox or forward something to a friend, Identity Chooser will open a window to force you to choose your sender address before it let's you access the email editor.
This workflow works every time and everywhere when 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.