A few weeks ago I wrote about An Interesting Safari Exploit. That has actually turned into a serious Microsoft Security Advisory. There is now code available on the internet to exploit these flaws, so anyone using Safari on Windows is vulnerable. From the information I have read, the prevailing recommendation is that people not use Safari until this can be fixed. There is more information and links to further resources at SANS Internet Storm Center.
I try not to force too many of my opinions on you, the good reader, but I do want to mention that I love Firefox. I don't have much of a need for any other web browser, and version 3 is just around the corner.