It looks like Firefox 1.5 changed the way the keypress event works. In previous versions, it only fired when character keys were pressed (as in IE), but in 1.5 it looks like it also fires for non-character keys, which is why backspace isn't working. I'm going to have to investigate and see what the best solution is.
Nicholas C. Zakas
Author, Professional JavaScript for Web Developers (ISBN 0764579088)
http://www.nczonline.net/