Grayscale image filtering for Mozilla

In trying to speed up my web pages, I want to avoid using multiple
images (disabled, hover, nohover). I've currently got opacity working
on hover and it looks great and works in IE and Moz. But I now need a
grayscale filter to show the image as disabled.

Does anybody know of a Mozilla way of doing this. I know that
-moz-opacity works for mozilla, is there an equivalent for grayscale?
Thanks,

JsD