As yet another security issue is found in Internet Explorer, it is a good time to remind you that Internet Explorer is used for more than web browsing. A few months back I pointed out how Microsoft Project had problems due to the ActiveX security settings I had set too high. Another such program is Outlook and Outlook Express.
With the latest issue, you could receive an email with embedded code that would exploit your computer. This problem is big enough that Microsoft may actually release it outside their normal monthly patch cycle -- they have only done this once -- to fix a problem with their DRM. This problem is big enough, you can actually get a fix from a third-party.
Perhaps a better fix, in addition to using Firefox (or Netscape) instead of Internet Explorer, use an alternate email program such as Eudora or Thunderbird (from the makers of Firefox).