Its due to the fact that the icons for these two widgets are in SVG format.
It turns out to be simply that Firefox's implementation of SVG doesn't (yet) extend to the <img> tag. It only works using <object>. However, given that Safari and Chrome et al support it I guess its just a matter of time before FF's SVG implementation catches up.
As for IE, it just doesn't support SVG. At all.
The W3C spec itself recommends SVG for icons as they can scale nicely for any sort of gallery, which is a good reason to use them, but maybe we have to use a fallback trick for IE and FF, as described here: