Go to the 'resizeImages' sample page in the latest Firefox (34, on my Mac):
http://cdn.mobify.com/mobifyjs/examples/resizeImages-img-element/index.html
If you do a 'shift-reload' of the page, none of the external resources on the page load up.
The best I can surmise is that Firefox is trying to load up those resources before the capture starts, then it aborts the loading during the capture, then when the page is rebuilt, Firefox doesn't try to load those resources again. Does this sound reasonable to you?
We're using a variant of the capture library (for our own nefarious means) and we've seen that every once in a while when a captured page is rendered (only on Firefox), some of the external resources don't get loaded up (but others do).