When the Mozilla foundation took over the Netscape codebase, it was initially called Mozilla, or Mozilla Browser. There was also a Mozilla email client that came from Netscape Communicator.

Then they made a trimmed-down version of the browser with only essential features. That was initially called Phoenix, then Firebird, then Firefox. They did the same with the email client and called it Thunderbird. These existed alongside Mozilla Browser for a while until it was discontinued.