In reply to Enty:
So that the image in your sig is clickable? It is, but I doubt it'd be a good idea since quite a decent percentage of clients (and email users) default to html off which would cause the image to fail to load. Prob makes more sense to just add a link under or above the image, since most email users wouldn't think to click on an image in a signature :x
Should explain how to do what you're after:
http://forums.mozillazine.org/viewtopic.php?f=39&t=584340
Post edited at 20:30