Direct from the Mozilla.org:
Mozilla 1.7 (Internet Application Suite)
Mozilla 1.7 is our mature, integrated application suite that includes browser, email and newsgroup client, chat client, and an HTML editor. Mozilla 1.7 is not a combination of Firefox and Thunderbird. Mozilla 1.7 is a completely different application.
We suggest Mozilla 1.7 if you are looking for an all-in-one solution when using the Internet. Large organizations that require an integrated suite (past Netscape Communicator users) should consider moving towards Mozilla 1.7. All others should consider upgrading to Firefox and Thunderbird.