Be advised that Firefox is an open source web-browser so whatever holes there are, they are probably known to hackers. Somehow I think that hackers won't try to exploit it that much since it's probably the browser they use :P.. It's available for free at
www.mozilla.org. It's a pretty slick browser, I've been using it for just over a year now, it takes a little getting used to, but once you get the hang of all the tricks and shortcuts and get it set up the way you want it, it's awesome.