There's a long standing bug/feature with Firefox with input type=image elements where user has images turned off, or the image is 404ed, or just not loaded...when it's clicked, no x/y coords are sent to the server.
Just this week, we've had complaints from users getting an error on our site they shouldn't. As it turns out, they get the error because we check to make sure button.x and button.y have values...but firefox is passing them without values.
Now, for the odd part. This is happening to people who by all appearances, have the image loaded. My firefox 2/3 work just dandy, yet we have one internal user who gets the error (but the image is loaded via a provided screenshot).