"According to secunia.com
Internet Explorer has 20 out of 79 security vulnerabilities that are still not patched in the latest version (with all vendor patches installed and all vendor workarounds applied),
while Firefox has only 4 out of 12 security vulnerabilities unpatched.
Based on information on secunia.com (1 and 2) we can see the benefit of an Open Source browser in the security field:
while Internet Explorer only issued a patch for 52% of the bugs found and applied partial fixes in 14%, Firefox has not only patched 69% of its flaws but it has never used a partial fix or a workaround. Quoting Marc Erickson: "Its Open Source nature means that anyone can look at the code and either find or fix holes - and development can go on 24 hours a day, as programmers in different time zones around the world wake up and begin their day.
24 hour development is extremely difficult for most proprietary software companies to do - they need to be very large - like Microsoft - and then they run into large corporation difficulties - politics, turf wars, who gets credit for accomplishments, project coordination, how does a boss in one time zone supervise employees around the world when he has to sleep, etc.
If we look at Secunia's criticality graphs (1 and 2) we can see that Firefox has 0% extremely critical and 8% highly critical bugs while Internet Explorer has 14% extremely critical and 27% highly critical bugs."
http://www.net-security.org/article.php?id=792