I read up on this a while back and i did mention it earlier in this thread, BUT, as firefox becomes more popular it will attract more and more attention from virus writers and malware.
It is actually easier for fiends to attack firefox as it is open source and so anyone can examine it minutely for exploitation points, also it does not have the backing to fix bugs/close loopholes that a company of Microsofts size does.
I saw an articlre that basically says all this and some more stuff, the fact of the matter is they BOTH have security issues and always will have.
Personal preference is the only way to go.
You think Microsoft gives a flying <censored> about bug fixes? Unless it is an issue which effects Win 2000/2003 internet servers and thus can have disasterous effects to most of the net, they don't really want to know.
Open source products will always have regular bug fixes, and not just the severe security holes. When it comes to the software OS debate, I am a fan of M$ products. But I have to be fair and say bug fixing is not a speciality of M$ at all.
As for the security flaws in Firefox, they pale in comparison to those of IE and Windows itself.