He's right. Mozilla, a $200-300 million a year outfit, currently maintains their software including Firefox. It's a huge C++ application. People who code XUL in their spare time, even a bunch of them, aren't likely to make a dent in keeping parity between a Firefox fork and the main release. They'd likely have trouble even porting it.
So, a fork is a rough solution and will have maintenance issues for an app this size.