I imagine Firefox would die instantly if it moved to a restrictive license. It would be too easy to simply switch to a popular fork, and Firefox's userbase are the type that would be compelled to follow through with it. Even ignoring FOSS principles reasons, most folks are browser-savvy enough to understand the implications of such news -- their favorite browser is about to kill itself so they have to pick another one.
Also consider that Firefox would no longer be the default browser in most Linux distros, and likely prohibited from official package repositories entirely.
