As a top case? It's awfully small for that . . . .
But if you're looking for a small top case, it mounts with the same monokey system used by the V46, E52, etc. Personally, at 20 liters smaller than my E52 (OK, nineteen liters!) I'd find it useless as a top case.
But for use as a side case, as it's designed, Givi does not offer mount kits for the FJR. I think they used to in Europe for the Gen-I bikes, but I'm not sure about that.