Firebird was the browser's name, after phoenix and before rebranding to firefox.

So it wasn't used before Thunderbird, that was the point of OP and myself. We were talking about the email client(!).

And I was an user of firebird, the database.

Firebird was actually the database whose name they hijacked when they had access to AOL's legal army.

Also K9Mail is now Thunderbird for Android.