Depending on your definition of "fork", Firefox has been forked in cases like the Tor Browser or IceWeasel. But starting a completely separate organization that

Depending on your definition of "fork", Firefox has been forked in cases like the Tor Browser or IceWeasel. But starting a completely separate organization that could keep its isolated Firefox fork competitive with Chromium would require a massive amount of resources. Mozilla employs hundreds of full-time engineers (although that number is declining).