I think you're conflating the Engine, Gecko, with the browser, Firefox. The Browser, Firefox, is easy to fork, judging by the multitudes of soft forks a la Libre Wolf, Zen, etc...

The Engine, Gecko, however, is hard to fork since it's tightly coupled to the browser itself.

I also think that when the parent mentioned "forking Gecko", it might be in the sense of extracting the engine and putting a new browser on top of it, just like other Webkit based browsers e.g. Orion and Gnome Web.