Do you remember the first time you accidentally sent a work related email from your private email account? Identity Chooser helps you to prevent that. It makes choosing the sender address explicit:
Do you want to compose a new email? Identity Chooser forces you to choose the sender address first.
Do you want to forward an email? Identity Chooser forces you to choose the sender address first.
To make it short: Every time you'll create a new message, Identity Chooser shows you a menu with all your identities.
Everything in Identity Chooser is about making explicit choices. No automatisms, no defaults.
And as a result you'll never send an email from the wrong email address again.
Head over to addons.thunderbird.net to find the current version. Development releases might be available earlier in the Releases section on GitHub.
Starting with version 4, Identity Chooser is a pure mail extension and doesn't use any experiment APIs.
This addon uses code from other projects: