system
5
Firebird is the browser only, no email client. It is a standalone version with some enhancements of the browser part of the mozilla suite which includes browser, email, web page composer. The standalone email program, based on Mozilla, is Thunderbird. IMO, Thunderbird is not as polished as Firebird. They both are technically still in development, but Firebird 0.7 can duke it out with IE 6 any day.