My own definition of furball:
A group of planes, around 5 or more, who are dogfighting just to dogfight, with the only objective being to shoot the other guy down. Furballs usually end up 5k or less.
For me the critical difference to call it a furball is the apparent objective: dogfighting for the sake of dogfighting. Doing it away from any bases (or sometimes, even near a base). A melee of planes flying around without a clear objective. This definition is only slightly different than a furball over a field, where your objective is to cap the field, or on the other hand, remove the cap. To me, classic furballing is fighter vs. fighter, with nobody trying to take a base or defend a base; just a gaggle of planes shooting it out. Little-to-no coordination on either side, every man for himself, with the occasional wingman tactic.
Furballs are fun, if that's what you are looking for in your game. I like them from time to time.
Fury