There are no black people playing FOnline so don't worry 
This for one.
And two, they use the word themselves all the time.
Don't give me some bullshit about not all of them do. Because they all have at least once, and I know for a fact the majority use it on occassion, having moved across my country to many places and all use this word regularly.
yet practically everywhere nobody gets pissed and defends the white man for being called a cracker, etc.
So yeah.
I have played with at least one black person on Fonline that I know of for certain. However that's not the point because what your saying is basically that it's OK to be racist when members of the race in question are not around.
I would say that while it's ok to joke with your close friends about your and thier race alike in ways that puts everyone at ease with each other by shrinking the elephant in the room into a humerous and non-threatening miniature, the exact opposite is true for getting together with people of only your own race in order to make fun of those from another.
The former brings us closer together by taking ourselves less seriously and equalizing us through neutralizing the minute genetic differences that cause such problems while the latter only widens the divide between us through separation and strengthens an inability to communicate like humans when faced with people from another race.
Words like 'cracker' are also offensive and racist if used in a offensive and racist way. But do we have that problem here? according to you if anyone used that word here that person would probably be white themselves, so who cares right!? It works the same way with the word in question when black people use it amongst themselves. The only diffence is that there is no history of black Africans enslaving white Europeans, certainly not on the industrial scale the reverse happened on, so I wouldn't compalin.
Don't give me some bullshit about not all of them do. Because they all have at least once, and I know for a fact the majority use it on occassion, having moved across my country to many places and all use this word regularly.
I don't care how many times you have moved, you have not met every black person in 'your country' and saying that every single one of them does anything besides grow skin different from yours is a gross generalization and in itself pretty racist imo.