Didn't work in Firebird @ 1600x1200 for me either.

I looked around on the Mozilla site and couldn't find anything specific about any 'bugs' that would cause pinup to render only the the one static size.
Maybe you could try putting a width measurement in? That wouldn't size properly, even if it did work, though. Can't think of anything else.
I don't want to see cyanide like that. Please.lol