It's always amazed me how awful Full Tilt security/customer support staff are at doing their jobs. Not that I think you were getting cheated OP, but it's almost comical how little they care if you were or not. Rather than address your specific concerns, all they ever do is mail out some irrelevant copypasta and wish you luck at the tables.
Anyway, I have to side with the people recommending you get your PC checked for Trojans ASAP. Also, try this experiment. Disconnect completely from the internet (physically unplugging your internet cable), and keep your browser open for a while and see if it still closes itself periodically. If so, it may be that you are just having troubles with the latest edition of Firefox. I had some problems with it myself and from what I've seen there seem to be a lot of bugs and issues with incompatible add-ons which cause it to malfunction. Speaking of which, I hope you are already a user of the
NoScript add-on for FF. You're a lot less likely to get hacked with this protecting your browser, even if you're one of those people that clicks on random links (not recommended).
Best of luck.