Secunia Graph of Advisories for Internet Explorer 6
See all the information pertaining to Internet Explorer 6
Secunia Graph of Advisories for Mozilla Firefox
See all the information pertaining to Mozilla Firefox and Secunia
Secunia Graph of Advisories for Opera 7
See all the information pertaining to Opera 7 and Secunia
Note: Internet Explorer takes the brunt of the advisories. That is to be expected as for the past few years it stole nearly 100% of the browser market. Of note as well is that Opera while having a much smaller user base has about 50% of the amount of advisories that rival Internet Explorer has.
What is noteworthy is to see that already Mozilla firefox is racking up a few. Will this trend continue remains to be seen. As it gains in popularity however, you may just start to see an increase in vulnerabilities similar to what you see in Internet Explorer now.