Between Netscape Navigator and Firefox, their web browser was called simply "Mozilla". It supported GUI themes in XML with images which were officially called "Chrome". Mozilla also hosted user-contributed themes on a web site called "Chrome Zone".
The browser was considered slow and bloated however, and when Firefox came, its lack of theme support was perceived as part of it having been de-bloated.
I might vaguely recall Mozilla being in an easter egg or alternate throbber in a Netscape browser, and my impression was that it had been an internal codename at Netscape which was then adopted for the open source project.