Internet Explorer 8
http://secunia.com/advisories/product/21625/
Affected By 9 Secunia advisories
32 Vulnerabilities
Firefox 3.5
http://secunia.com/advisories/product/25800/
Affected By 7 Secunia advisories
54 Vulnerabilities
Firefox 3.0 is even worse!
http://secunia.com/advisories/product/19089/
Affected By 23 Secunia advisories
150 Vulnerabilities
Most other browsers have much fewer documented exploits ... One can't help but wonder if this has something to do with the fact these browsers are the most used, the design of these browsers, or a combination of both ... Also remember, the above list does not include all the firefox extensions, and I saw any advisories for many different firefox extensions too ...