The simple answer to multiboxing is that yes it is allowed.
The long answer is that you cannot use macros, bots, programs, or addons to control a character that is being used by one user on Firestorm. So if you have, let's say, 5 accounts that you are multiboxing they can only be "following" your main character unless you are directly controlling another character that is multiboxing. That means if you are doing anything other than "follow" on a character that you are not controlling with direct input from yourself, it is not allowed.
Firestorm has made this clear through a single rule:
Automated Keypresses: Using a macro, keyboard addon or multiboxing tool assist to automatically press a key in any circumstance if forbidden. Some automated addons used for harassing players, harming the server or advertising things will be punished according this rule as well.
This includes multiboxing as stated in the rule.
But then you have people like me, who buy things out of the AH and repost for higher profit. You can't say that someone is a bot because they sell a large quantity of items. That's not proof. If I buy all the green/blue gems out of the AH because people are selling too low, and sell 10k uncut gems does that mean I'm a bot? No it doesn't. It means I have gold to do what I want.