@Ken Lord

Yes, vulnerabilities will turn up in Firefox just as in Internet Explorer, and yes, as Firefox becomes more popular it will become a target for criminal hackers. The difference is that when a vulnerability is found in Firefox, the open-source community gets on it right away so it is usually fixed within 24-48 hours; while Microsoft usually takes weeks or even months to sort out vulnerabilities in Internet Explorer. So even if Firefox becomes the majority browser (speed the day!) I would still use it for that reason alone, notwithstanding its customisability and huge range of useful plugins.