feComposite in Firefox

Hi all,

I thought I had figured this out once, but can't seem to find a working example.

As you may know, Firefox doesn't implement backgroundImage
http://cs.sru.edu/~svg/Chapter05/G05xx45.svg  (though Opera, IE and ASV do). I'm not sure if this is a philosophical objection (like SVG fonts) or just happenstance.

It also doesn't allow feImage to refer to document fragments
http://cs.sru.edu/~ddailey/svg/feDisplacement4.svg (though Chrome and Opera do)
(see https://bugzilla.mozilla.org/show_bug.cgi?id=455986 ). The existence of an open bug would suggest it is not a philosophical objection.

Can someone think of another way to implement feComposite in Firefox with these particular hands held behind one's back?
The page at https://developer.mozilla.org/en-US/docs/Web/SVG/Element/feComposite doesn't seem to include an example.

cheers
David