Got this piece of Mootools code that seems to not be working in Firefox. I'd prefer to use jquery, but I came into this situation after the site was built, looking for a fix rather than a replacement. If you look at the screenshot, you can see the custom checkbox and a link that you rollover to see a preview image. Works fine in IE, Chrome, Safari, but not Firefox!
try to define dimensions of your checkbox
Couple things did it - needed complete input tags (original code monkey forgot to include the id and for tags), and to define dimensions. I also switched it with a jquery solution to be sure, but I'm fairly certain it would've worked with Mootools had the input tags been coded properly.