Using canvas here in doesn't render the zoomed view when apzc zoom is applied. http://mxr.mozilla.org/mozilla-central/source/testing/specialpowers/content/specialpowersAPI.js#1187 This will make debugging tests hard, and will probably break other things related to thumbnailing.
The gfx team has bug 744100 on file for a compositor API to capture what is displayed on-screen. That should be a better solution for this problem.
Given the metro use case is no longer being actively pursued and there's already a general purpose gfx bug for this I'm just going to dupe this to that.