I have seen so many forums complaining about it, yet I have no idea what it is? Is it like Gold Farming in WoW? How can I tell if somebody is farming in Warzone? Is it in all modes or only Firefight and Assualt? (I hope not, as I have been playing Firefight a ton since I found out 343 brought it in)
There isn't farming in firefight.
What farming is in Warzone involves triple capping you and then ignoring the objectives in order to spawn kill you as much as possible. The more dedicated farmers will have people running around the core room with a sword, two guys with scorpions sitting outside, and snipers watching the exterior spawn as a few examples. I've seen a couple instance where the opposing team will only capture the near base so that you have no choice but to spawn at your home base in their line of fire and that way they get less points from holding the bases.
In WZA the defending team will set up a spawn kill and then let one member of the attacking team up to capture a base when the timer is about to go. They'll then set up the spawn kill on your spawns for the second objective and when the timer runs low let someone up to capture the second base. So that they can extend the spawn kill further.
It's extremely unpleasant and will likely only be solved by team based matching.