Historically, Mozilla had been used internally as a codename for the Netscape Navigator web browser from its beginning... ...The name was created as a contraction of the words "Mosaic killer", hinting that Netscape would be the end to the (then only) competitor browser, Mosaic.
About Mozilla Firefox
The Firefox project began as an experimental branch of the Mozilla project by Dave Hyatt and Blake Ross. They believed the commercial requirements of Netscape's sponsorship and developer-driven feature creep compromised the utility of the Mozilla browser.