FF tubes heat up badly after more than a couple of mags, to the point where you'd need an M60 asbestos glove to hold them.  They don't allow access to the barrel or gas tube.  Barrels must be oiled in the field or they will rust, and gas tubes can warp or melt from extended high rates of fire.  Getting mud out of a FF tube isn't any fun.  Basically, FF tubes are intended for bench use, not field use.  The bit of extra accuracy is meaningless unless you are bench shooting anyway.

Accuracy Speaks makes carbine-length DCM-type float tubes (the kind that accept regular handguards).  They add a bit over a pound of weight, and have most of the problems above, so again, I'd recommend against them for field use.

If you must have a FF tube, look into the Knights FF RAS (hard to get right now; they're new) or one of the other new rail systems coming out, like the ARMS SIR.  They address some of the above issues at least.

-Troy