You can normally add it by copying the icon into your buffer and then opening the info window for Firefox (get info, after clicking on the firefox icon in the folder its installed in), and then clicking the top copy of the icon in the window and clicking paste.
BUT...I have a question. I can see that this image has the transparency of the background properly defined, but when I save it to my desktop, it shows up in the preview with a black background, and if I paste it in as an icon, it also shows up as an icon with a black background. In Preview, it has a white background, like it should (since its transparent, and the preview background is normally white).
So why isn't it transparent as an icon, in the dock, etc? I've had this problem a few times now....