The CEO of Mozilla, John Lilly, has come forward to voice direct support for the EFF's cause, saying that allowing a jailbreak is a choice, a good one at that, which shouldn't come with the stigma of being criminalized. Whether or not Mozilla's support has anything to do with the development of a mobile Firefox browser isn't mentioned, but it is clearly possible. Apple has prevented applications from appearing on the App Store multiple times that “compete” with something Apple provides. It doesn't take much imagination to picture Apple denying Mozilla a Firefox release for the iPhone, claiming it competes with Safari.
Mozilla isn't the only software development company that is backing the EFF in their efforts against Apple. Other companies, such as Skype, have been vocal about their discontent with Apple's politics surrounding the iPhone, whether it is their App Store or the platform restrictions. Regardless of what Apple's stance on the matter continues to be, however, they'll never be able to stop people from jailbreaking the phone.