[amiga386]

That is the case, but even then Firefox really was a fork, within Mozilla.

Mozilla was created in 1998 to open-source Netscape Communicator suite. Mozilla released its own suite, also called "Mozilla" (e.g. "Mozilla 1.0" [0])

Independently of that effort, Dave Hyatt and Blake Ross made an experimenal, cut-down version of just the browser part of the suite, which they called "Phoenix", as in a Phoenix rising from the ashes. That's a fork. That's a fork by any metric.

They later rebranded Phoenix as Firefox, and eventually the Mozilla suite was abandoned. Mozilla changed tack in 2003 and switched to developing Firefox and Thunderbird as independent products [1]

[0] https://www-archive.mozilla.org/releases/mozilla1.0

[1] https://www-archive.mozilla.org/roadmap/roadmap-02-apr-2003