Can I transfer gold from Firebox to a USA server?
Depends on the server. Gold transfer is only possible within one domain (COM to COM, DE to DE, etc). And only Gold you bought is transferable. The Gold you won in the game is not transferable. If you have transferable and not transferable Gold you can buy Plus with the not transferable Gold. Plus day will be transfered too if at least 1 transferable Gold is existent.
Firebox is COM 2 server, and the gold can't be transferred to any of the US servers (US 1 Crankshaft, US 101 Golden Gate, US 102 Grand Central, US 103 Broadway)
It can only be transferred to other COM (international) servers, and there are plenty of those