Mozilla is an organisation that creates the browser Firefox. However, it used to create the Mozilla Suite [1], a set of applications that included e.g. a browser and an email client, which evolved into Firefox and Thunderbird respectively. SeaMonkey is somewhat of a successor to the Mozilla Suite [2].
[1] https://en.wikipedia.org/wiki/Mozilla_Application_Suite
[2] https://www.seamonkey-project.org/