Custom Cursor Image CSS

I have a number of heroshot images, that have a modal popup when clicked. I'm trying to get my cursor to turn into magnifying glass whenever it is moved over the image. The following CSS does not appear to work even though my magnify.cur is present in the right location.

a.heroshot img {
  cursor:url(/img/magnify.cur), pointer;
}

Has anyone ever done anything similar? I don't mind a JavaScript solution if one exists.

EDIT: It works in Safari, but not in Firefox.