See bug 789707 comment 5. I've seen this before, probably when I've had connectivity issues. We should probably cache the icon locally somehow, or at least for now just specify the iconURL as a data: URI.
I don't think we have to do anything extra to specify them as data:URI, just update the manifest accordingly.
Fixed by bug 801951.
[qa-] along with bug 801951. Please add verifyme keyword if there's something QA needs to test.