You'll get 5% fire dmg yeah. The 15-25% is for skills and their procs as specified, so you don't get that.
Actually the 5% fire dmg is added based on your "base dmg" (normal weapon dmg and rings/amulets), not fire-based skills. And it also doesn't work for the "elemental" part of weapon dmg.
I don't remember the source of this info, I just recall reading it on the forums (and I think we even had a blue post about this).
The 15-25% is for any "abilities" (and I assume procs as well, as long as they deal the specified dmg - because that's how Fragment of Destiny works for Wiz skills at least), so if anything that could affect the Fireball.