Yeah, Tritium has it right. Full Range, Flat Response. I too never heard of "FRFR" until a couple years ago. Me and the folks I've dealt with always just referred to it as "full range". After all, if you were making a two-way or three-way full range speaker it was probably for sound reproduction where the goal was to be relatively flat.
The only new concept to me was the GRFR, or "guitar range flat response". That involves a single speaker that is relatively flat up to typical roll-off frequencies for guitar. That solution only makes sense for modeling/profiling, since the typical non-flat guitar speaker is a major part of the guitar sound. A normal guitar amp without a cab-sim would sound terrible for most players with this type of speakers.