Comment 18 for bug 162993

http://mxr.mozilla.org/seamonkey/source/modules/libpr0n/decoders/icon/gtk/nsIconChannel.cpp

Can anyone here who has an x64 machine add some debug symbols or printf's to their build and say exactly which path the icon rendering is taking? It might be returning one of the error codes.

This is a typical stock icon URL that can be used for testing:
moz-icon://stock/gtk-go-back?size=toolbar