That's not a very helpful response - what's the Firefox security setting that might trigger the website's programming? It's not the browser that logs you off, it's the website. I think that their support should give you more of an idea here, but maybe they don't know.
At any rate, I'd see what happens with IE, Opera or Safari. That might give you a clue.
[edited by: tedster at 5:46 pm (utc) on Jan. 4, 2008]
Then it's not the browser setiings - something else is going on. Perhaps your IP address is being denied? At any rate, giving their tech support the added information that this happens with all those browsers should kick them into a clarified response.