Watch for http://127.0.0.1 vs. http://localhost. The two are not the same from Mozilla's perspective. I spent hours trying to troubleshoot a non-existent problem all because I was referencing 127.0.0.1 in the code and localhost in the browser. Mozilla would throw an exception: "Permission denied to call method XMLHttpRequest.open" whenever I tried do an xmlrpc request from Mozilla using the jsolait libraries. The same code had worked previously without monkeying around with signed.applets.codebase_principal_support but was failing because I was pointing the browser at localhost!