Thunderbird allows you to use different identities for an email account. You can do this in order to use different formatting, signatures, reply-to addresses, templates, encryption, etc for different circumstances. For instance, if you have another email domain attached to your GMail account, you can add another identity for that domain. You can then choose which identity to send from when composing a message, as if you had several email accounts added to Thunderbird.

It's quite easy to configure:

1. Open the View > Accounts settings in the main window.


2. Near the bottom of the dialog that appears, click "Manage identities"


3. Edit the preferences for that identity.


4. Compose a new message and notice that From can be changed when you write a message. When you do, all those preferences will be applied to the message.