On the one hand there isn't anything inherently "white" about Johnny Storm, what cultural traits WOULD make him or any character have to be white? Being "white," as far as the United States is concerned, is just being "American," but just being American is something any ethnicity can do. There's nothing about Peter Parker's situation that means he HAS to be white, nor Captain America's (hell, maybe being non-white would give them more reason for rejecting him for military service in the first place). What's left to justify a character's race as being white? Being racist? Or a Nazi? Evidently literally being a part of Saxon mythology isn't enough reason to keep a character white because it's okay as long as the actor is good (I know there's the whole justification that Asgardians are aliens in Marvel, but a similar reasoning could be applied to the Last Airbender, where they're completely fictional setting with people of no predetermined ethnicity, and that never flies with anyone for some reason).
On the flipside, the argument that it's okay for Nick Fury to be black because he was black in the Ultimate comics is bullshit, because that STILL means that in one adaptation his race was changed. Changing a character's ethnicity for a new comic book retelling is the same as changing it for a movie retelling. If you're fine with Nick Fury being black, you should be fine with Johnny Storm being black.
On the flipside, the argument that it's okay for Nick Fury to be black because he was black in the Ultimate comics is bullshit, because that STILL means that in one adaptation his race was changed. Changing a character's ethnicity for a new comic book retelling is the same as changing it for a movie retelling. If you're fine with Nick Fury being black, you should be fine with Johnny Storm being black.