Beside of Firefox (or Deer Park), there is one more Gecko browser on MacOSX. Camino is the name. For short, it is Gecko engine + Cocoa for interface. Firefox is Gecko + XUL interface.
You may spot the difference. Firefox advantage is cross platform application and easy-to-develop extensions. However, Macintosh users (I'm one of these) want a better integration between browser and OS, Firefox cannot provide this because of XUL. The more integration with some OS, you code is less portablitiy to another OS.
Camino is the solution. Camino developer use Apple Human Inteface Guideline carefully and Camino becomes more Mac-friendly than Firefox.