Scanit, a Brussels-based security company, has put online a Browser Security Test which tests for vulnerabilities in your web browser. There is a complete listing of the tests they run should you be interested.

I ran the test on 3 of my the big browsers (for the Windows platform, that is) and collected the results.

Results for Firebird 0.8.0+ (20040202) WinXP:

The Browser Security Test is finished. Please find the results below:
High Risk Vulnerabilities 0
Medium Risk Vulnerabilities 0
Low Risk Vulnerabilities 0

Results for Opera 7.23 WinXP:

The Browser Security Test is finished. Please find the results below:
High Risk Vulnerabilities 0
Medium Risk Vulnerabilities 0
Low Risk Vulnerabilities 0

Results for IE 6.0 WinXP:

The Browser Security Test is finished. Please find the results below:
High Risk Vulnerabilities 0
Medium Risk Vulnerabilities 0
Low Risk Vulnerabilities 0

Seems like all my browsers are safe. However, as this thread at SitePoint Community Forums shows, IE needs to be patched with the latest security patches (as mine has been) to protect it from severable major security flaws. If you haven't, do so now by going to Windows Update in an Internet Explorer browser!