It's not the header (I'm working on an Emerald hack btw, not FR). I've had RHEA for some time, but never used it other than to check for this very problem earlier. The header is BPEE just as it should be.ô.O
Of course, US version of Firered is supported. I use it myself with sappy.
Maybe you changed, accidently or not, the header code of the rom and now sappy can't load it.
Can u open it in AM, UnLZ, cyclone etc.?
The code I mean is BPRE, which is the normal one and says the programm: Hello, I'm this version, so load this data.
It can be changed with the header editor or whatever it may be called, or through hex. If it's changed, tools wont recognize which version it is and will say: Unsupportet or Unknown.
So maybe you changed it accidently, I don't know. Just try opening it with other tools.
edit: Download the Rom header editor advance from -hackmew-'s toolbox--> Here
Open the rom, and write BPRE into the game code box.
If there's already BRPE inside..dunno, just delete it and write it again and save.
I hope that's the problem.
I've already made quite a few changes using all kinds of tools. They all support it just fine.