Revealed by running 330010-1.xul in crashtest-ipc mode once necko security checks are turned on (bug 782542). Looks like RDFServiceImpl doesn't provide a nsITabChild in for HTTP channels it launches callbacks (in the test this is while loading http://localhost:4444/1355997828641/1/330010-1.rdf), and it thus fails necko security tests (and causes process to get killed). For now I'm simply disabling the test in bug 782542 in e10s mode. We can WONTFIX this bug if we think we're not going to need RDFService in child processes, otherwise it needs to pass in a TabChild.