FX firms make money "lending funds to customers" in the case of FXCM they charge $1 per mini lot for overnights(on wednesdays it's 3 times the normal rollover fee to account for the weekend)) unless you choose to be restricted to an account with a higher margin requirement (50:1). With 100:1 you don't get interest on rollover either if you are long the currency with a higher interest rate. You have to dig a little bit to find out about some of these "catches". On these firms sites they will only tell you how great FX is, "especially if you come for equities", 24 hour market, no commissions, earn interest, huge moves and liquidity, daytraders dream bla bla.... Check out the recent ET chat hosted by a purported FX trader promoting FXCM and advising you to trade off the 5min and15 min.chart. Bwahaha
Beware of stops. They are triggered not on sales but on bid/ask, I saw a stop taken out in a flash, without even seeing a bid at my trigger, their market was 3 pips higher and then bounced right back.