All versions of Windows since Win98 are vulnerable. Firefox users are still vulnerable if they have caching turned on (and 100% of everyone does), because the image is still stored on the hard drive and as soon as you access it, oops you're boned.

You're not automatically invulnerable to everything just because you use an alternative browser. The best Firefox users are the ones who recognise Firefox's weaknesses.

Microsoft have released a patch for this. Get it through Windows Update.