Actually, I believe all GBA Pokemon games use FF as free space. Or another theory, 00 is "temporary" free space while FF is completely unused data.
And no, Emerald does NOT use FireRed as a base. That's all Bulbapedia bulls#1t. I seriously doubt that GameFreak bothered re-mapping and re-scripting the entire thing. Otherwise, Emerald would have had animated FR/LG sprites for Kanto Pokemon, and it would have used FR/LG's dual text boxes, which it didn't. Even though some features were ported over from FireRed such as Berry Crushing and Union Rooms, I believe that Emerald uses FireRed as a base.
However, I think it's pretty obvious that HG/SS is built off of Platinum, after all, there's plenty of Platinum leftovers. Emerald doesn't have any FR/LG left overs, because it probably used R/S as a base. (though what's interesting is how Emerald contains the entire FR/LG soundtrack).
EDIT: lol, there's kinda no point for Team Emerald anymore.