I was having an issue with it not working in FF as well, but I got it working when I filled out the initial src attribute value with an image, like Monsieur Tuupola does on this example page, where he uses a 1px x 1px gray gif image for a place holder:
http://www.appelsiini.net/projects/lazyload/enabled_gazillion.html
Fire 'er up in FF, and it'll work.
