What the exploit will do to your PC depends on what the payload is that is used in the particular variant. It doesn't automatically trigger in Firefox I believe - you have to open the image using the PCs preview function. But if you have a desktop search application on your PC then the image will be opened and trigger the exploit when it is catalogged, which I believe will happen whether you viewed it in IE or FF as both have caches.

Best thing to do is to use Secunia, FRSirt, securityfocus, SANS isc etc... to keep an eye on the situation. Microsoft will come up with something..... probably before Christmas.