If you've associated an e-mail address with a public key, that address will be available to whomever you send that key or any signed messages to. This is so that the recipient can know which key to use when verifying your signatures, or when sending encrypted messages to you.
Multiple e-mail addresses can be handled in one of two ways:
Add additional addresses to your existing key. Of course, all e-mail addresses associated with the key will be available to any recipients of your key or signed e-mails.
Create separate keys for addresses that you do not want to be associated together.
To add addresses to your existing key:
- In Thunderbird, go to OpenPGP->Key Management.
- Right-click the key you want to edit, and select Manage User IDs.
- Add your e-mail address(es) in that dialog.
To create a separate key:
- In Thunderbird, go to OpenPGP->Key Management.
- Click Generate->New Key Pair.
- Use that dialog to generate a new key, the same way you made your first one - just select a different "Account / User ID".