This bug has to do with the way Firefox handles an ImageButton with no Image.
Add an ImageButton to a page. Add some text to the AlternateText property of the ImageButton. Leave the ImageUrl property blank. Add an event handler to the ImageButton Click event.
Compile the page and view it in both IE and Firefox. Click the ImageButton to generate a postback. Note that the Click event is fired in IE but not in Firefox.
Now turnon the Trace property for the page so that you see the Form Collection. Note that in IE, ImageButton1.x and ImageButton1.y both have values while these values are empty in FireFox 1.0.
Hopefully Firefox will fix this in their next release.
PS: It looks like if the image is missing from an input type="image", firefox will not render the area that the image is supposed to occupy and renders the alt text if it is available. When this input is clicked, no x and y coordinates are generated. In IE, a default area is rendered with a broken image icon and so x and y coordinates are generated.
UPDATE: Firefox developers do not see this as a bug. Here is why.