Secunia, well known for their vulnerability research and vuln announcements, has released the Secunia 2008 Report as pdf.
The report shows that they release more advisories per year and 0-day vulnerabilities felt back to the 2006-level for decreasing about 40 percent. Also very interesting was the comparison of browser security. While Firefox has more vulnerabilities than IE, Safari and Opera together, those security holes are closed way faster than the ones from IE. This makes Firefox in summary more secure.
My take: I recommend Firefox to everyone! ■