"There is one caveat: Symantec counts only those security flaws that have been confirmed by the vendor. According to security monitoring company Secunia, there are 19 security issues that Microsoft still has to deal with for Internet Explorer, while there are only three for Firefox."

However, it's not clear from the report how quickly Microsoft and Mozilla released patches for their respective vulnerabilities, or how many of the vulnerabilities were targeted by hackers, though Microsoft generally releases patches only on a monthly basis.