Userscripts.org

Just an idea, inside the very bottom of the main for loop, I added this:

if(img.naturalWidth == 0)
{
img.src = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8%2F9hAAAAAXNSR0IArs4c6QAAAAZiS0dEAP8A%2FwD%2FoL2nkwAAAAlwSFlzAAALEwAACxMBAJqcGAAAAAd0SU1FB9oBGA86J1xD7wYAAAAZdEVYdENvbW1lbnQAQ3JlYXRlZCB3aXRoIEdJTVBXgQ4XAAABvElEQVQ4y5WSvYpUQRCFv77TLLjCMoKCGLjKrpGCIIiBCIuh4EPoIwiGvoCBaCZoaGhkZqZPsIiJIMYqBhs4O%2Fd2d%2F0YdN%2FxDrMGVlRF852uc7pDSokPh798a%2BakZc%2Fxome5WLL4PbBc9CwXAzkVrt7Y5%2B7tvTCfbzOWuxNDCAQRDm7t8q969PIz2%2BfnvH73xR%2FcuxLOntvB3Qkh0OFQUtqAHMgGvdT52qUz%2FLx8ncevDv3H9yMCARw6gJzLhkBREIWsdT4VOx7uZQ5u7vPs%2BXsHIAQiQB7yGpwVxKDYX4Gnb79tenMnQqCkvFq7NDhb7YvC%2FTu7KGBAf6x8%2FPqpKQQiOKllMMJFq8Bo4eKFHbJAciiny1pS1UIqGCfD4lAEBgHr6vm0oqqSkiDmZAuUiXdx6s2tFwGt8aGq9R9426CMnq1CIzw0KCtoADFf5eVQM1AR8glw0goXBTVIgOn0pzjRDaRoTX4Kj32Ds0J2CL72ikTHKWIrz0kmQuPNY6CA69oCRDdvIVY4TTxLg1fZOHRNwMwxsxqiiEEMBIM4AzeIBiowa7MpRKBjqy1QvUQ3w8x58eQN%2F1NmjpvxB8Zda0vCfHXQAAAAAElFTkSuQmCC";
}

That displays Firefox's default favicon for sites that don't have one instead of just nothing.

The full URI (with line breaks you'd have to remove) is:
data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8%2F9hAAAAAXNSR0IA
rs4c6QAAAAZiS0dEAP8A%2FwD%2FoL2nkwAAAAlwSFlzAAALEwAACxMBAJqcGAAAAAd0SU1FB9oBGA86
J1xD7wYAAAAZdEVYdENvbW1lbnQAQ3JlYXRlZCB3aXRoIEdJTVBXgQ4XAAABvElEQVQ4y5WSvYpUQRCF
v77TLLjCMoKCGLjKrpGCIIiBCIuh4EPoIwiGvoCBaCZoaGhkZqZPsIiJIMYqBhs4O%2Fd2d%2F0YdN%2
FxDrMGVlRF852uc7pDSokPh798a%2BakZc%2Fxome5WLL4PbBc9CwXAzkVrt7Y5%2B7tvTCfbzOWuxND
CAQRDm7t8q969PIz2%2BfnvH73xR%2FcuxLOntvB3Qkh0OFQUtqAHMgGvdT52qUz%2FLx8ncevDv3H9y
MCARw6gJzLhkBREIWsdT4VOx7uZQ5u7vPs%2BXsHIAQiQB7yGpwVxKDYX4Gnb79tenMnQqCkvFq7NDhb
7YvC%2FTu7KGBAf6x8%2FPqpKQQiOKllMMJFq8Bo4eKFHbJAciiny1pS1UIqGCfD4lAEBgHr6vm0oqqS
kiDmZAuUiXdx6s2tFwGt8aGq9R9426CMnq1CIzw0KCtoADFf5eVQM1AR8glw0goXBTVIgOn0pzjRDaRo
TX4Kj32Ds0J2CL72ikTHKWIrz0kmQuPNY6CA69oCRDdvIVY4TTxLg1fZOHRNwMwxsxqiiEEMBIM4AzeI
Biowa7MpRKBjqy1QvUQ3w8x58eQN%2F1NmjpvxB8Zda0vCfHXQAAAAAElFTkSuQmCC

[Edit] On second thought, that may not be the best way to detect if an image has loaded or not.
It may have simply not loaded yet, but will finish loading in the next split second.

A dirty fix would be to give each icon an id:
img.id = "favicon" + i.toString();
and then, at the bottom, instead of what I wrote above:
setTimeout("var img = document.getElementById('favicon" + i.toString() + "');\
if(img.naturalWidth == 0){img.src = 'the nightmare from the previous post';}", 750);

Just throwing out ideas.