Not true. The original Mozilla Internet Suite was a combined browser, email/newsgroups client, and web-page editor. It was an open-sourced version of Netscape Communicator which did the same things. Seamonkey is the modern fork.
Firefox is the result of a project to cut down the Mozilla suite to something smaller, simpler and more focused, but its parent program and direct ancestor edited text, HTML, and did other read/write stuff like messaging, calendaring, etc.