Schneier on Security: Internet Explorer Sucks: Security God Bruce Schneier tells it like it is.
MSIE was 98% unsafe. There were only 7 days in 2004 without an unpatched publicly disclosed security hole.
Firefox was 15% unsafe. There were 56 days with an unpatched publicly disclosed security hole. 30 of those days were a Mac hole that only affected Mac users. Windows Firefox was 7% unsafe.
Opera was 17% unsafe: 65 days. That number is accidentally a little better than it should be, as two of the upatched periods happened to overlap.