I am going to post this in the browser thread, but I did a test of the three browsers on a website that I test security on. I can't post the link because it is not a legal site, but that's not the point. Every single page and download on the site are chock full of self-installing spyware, virii and trojans. Tons of them.
So, I first tested antivirus programs on it. The only one that actually did anything was Avast! In fact it was so bad that Avast! shut the website down so the server connection was dropped and you couldn't view the page.
Then I tested all three browsers on it. ALL OF THEM let the malware through. Every single one. None of them stopped it. Which proved conclusively that ActiveX is not the threat anymore, Opera and Firefox are every bit as vulnerable to infection as IE7 is, and it is completely pointless to cripple the web by viewing it with Opera or Firefox. You won't be doing yourself any favors.
If you want the fastest browsing, both IE7 and Opera smoke Firefox for rendering. Firefox screws up on the rendering of some pages because they are coded for IE. There really is no sense in getting it from my testing.