Too many un-addressed vunerabilities in FF. Stick with IE. It has it's issues but at least you're guaranteed fixes.
..unless you couple NoScript to FireFox surely?
..and that IE is still the most popular browser, so most attacks will be directed at that instead? I've always found Mozilla get patches out pretty quickly.
Most vulnerabilities would be started by scripts (Javascript or ActiveX), and if you stop the scripts from starting, then surely they can' t do anything?