Firefox is a decent browser for home use, but it has serious flaws. It has had many significant vulnerabilities, including several that had been previously patched in IE. Talk about not learning from others mistakes. They also don't have a clear strategy for patching vulns and notifying users. And reinstalling the entire browser to patch a hole is asinine.
Also, the oft-mentioned argument about many eyes looking at the code for holes is crap. The vast majority of people looking at the code wouldn't know what a vuln looked like if it hit them, and those that do would rather work on sexier things than looking for vulnerabilities.
Not to mention FF's utter lack of enterprise managment with Group Policy. How is an admin supposed to configure a proxy server and a proxy exception list for thousands of clients and prevent the users from changing those settings? That can be done in five minutes with IE and GPOs, along with every other setting in IE..