Firefox is a perfectly nice piece of software, but it’s getting more hype than it deserves. It’s not the panacea that will keep you safe; it has its own security issues and it will be targeted by the bad guys just as Internet Explorer has been targeted.
Here’s an article that discusses the security problems already identified in Firefox, which currently has a haphazard way of notifying users and issuing updates. I also have run into bugs caused by a badly designed uninstall routine for Firefox, requiring trips through Microsoft support documents before systems were able to behave normally again. It’s not a bad program; in some respects it’s quite a good program. When you consider it, though, beware of evangelists that overstate its virtues!