I think I figured out the problem with bug 384192; the code added to quit.js is also used in the browser tests, but it triggers an undefined-variable exception there, and I think the tests are getting killed by test timeout code that doesn't print a helpful 'tests timed out' message to give a little help determining the cause of the failure. We shall see...

dom/tests/mochitest/dom-level0/test_crossdomainprops.html

@@ -12,7 +12,7 @@
   
 </div>
 <iframe name="frame1" src="file_crossdomainprops_inner.html"></iframe>
-<iframe name="frame2" src="http://localhost:8889/tests/dom/tests/mochitest/dom-level0/file_crossdomainprops_inner.html"></iframe>
+<iframe name="frame2" src="http://example.com/tests/dom/tests/mochitest/dom-level0/file_crossdomainprops_inner.html"></iframe>
 <pre id="test">
 <script class="testbody" type="text/javascript">
 

testing/mochitest/runtests.pl.in

@@ -359,9 +359,6 @@ sub initializeProfile {
   my ($app_path, $do_browser_tests) = @_;
   my $pref_content = <<PREFEND;
 user_pref("browser.dom.window.dump.enabled", true);
-user_pref("capability.principal.codebase.p1.granted", "UniversalXPConnect UniversalBrowserRead UniversalBrowserWrite UniversalPreferencesRead UniversalPreferencesWrite UniversalFileRead");
-user_pref("capability.principal.codebase.p1.id", "http://localhost:8888");
-user_pref("capability.principal.codebase.p1.subjectName", "");
 user_pref("dom.disable_open_during_load", false);
 user_pref("dom.max_script_run_time", 0); // no slow script dialogs
 user_pref("signed.applets.codebase_principal_support", true);
@@ -369,6 +366,49 @@ user_pref("security.warn_submit_insecure", false);
 user_pref("browser.shell.checkDefaultBrowser", false);
 PREFEND
 
+  #
+  # Since some tests require cross-domain support in Mochitest, across ports,
+  # domains, subdomains, etc. we have a proxying hack in cross-domain.js which
+  # maps a bunch of servers onto localhost:8888.  We have to grant them the same
+  # privileges as localhost:8888 here, since the browser only knows them as the
+  # URLs they're pretending to be.  See cross-domain.js for the other copy of
+  # this list.
+  #
+  # KEEP THESE TWO LISTS IN SYNC!
+  #
+  my @servers = ("http://localhost:8888",
+                 "http://example.org:80",
+                 "http://test1.example.org:80",
+                 "http://test2.example.org:80",
+                 "http://sub1.test1.example.org:80",
+                 "http://sub1.test2.example.org:80",
+                 "http://sub2.test1.example.org:80",
+                 "http://sub2.test2.example.org:80",
+                 "http://example.org:8000",
+                 "http://test1.example.org:8000",
+                 "http://test2.example.org:8000",
+                 "http://sub1.test1.example.org:8000",
+                 "http://sub1.test2.example.org:8000",
+                 "http://sub2.test1.example.org:8000",
+                 "http://sub2.test2.example.org:8000",
+                 "http://example.com:80",
+                 "http://test1.example.com:80",
+                 "http://test2.example.com:80",
+                 "http://sub1.test1.example.com:80",
+                 "http://sub1.test2.example.com:80",
+                 "http://sub2.test1.example.com:80",
+                 "http://sub2.test2.example.com:80");
+  my $i = 1;
+  my $server;
+  foreach $server (@servers) {
+    $pref_content .= <<SERVERPREFEND;
+user_pref("capability.principal.codebase.p$i.granted", "UniversalXPConnect UniversalBrowserRead UniversalBrowserWrite UniversalPreferencesRead UniversalPreferencesWrite UniversalFileRead");
+user_pref("capability.principal.codebase.p$i.id", "$server");
+user_pref("capability.principal.codebase.p$i.subjectName", "");
+SERVERPREFEND
+    $i++;
+  }
+
   my $chrome_content = <<CHROMEEND;
 \@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */
 toolbar,

testing/mochitest/server.js

@@ -42,9 +42,7 @@
 // sucks.
 
 const SERVER_PORT = 8888;
-const SERVER_PORT_OTHER_DOMAIN = 8889;
 var server; // for use in the shutdown handler, if necessary
-var otherDomainServer; // for use in the shutdown handler, if necessary
 
 //
 // HTML GENERATION
@@ -138,16 +136,12 @@ function runServer()
   serverBasePath.append("mochitest");
   server = new nsHttpServer();
   server.registerDirectory("/", serverBasePath);
-  otherDomainServer = new nsHttpServer();
-  otherDomainServer.registerDirectory("/", serverBasePath);
 
   if (environment["CLOSE_WHEN_DONE"])
     server.registerPathHandler("/server/shutdown", serverShutdown);
 
   server.setIndexHandler(defaultDirHandler);
   server.start(SERVER_PORT);
-  otherDomainServer.setIndexHandler(defaultDirHandler);
-  otherDomainServer.start(SERVER_PORT_OTHER_DOMAIN);
 
   // touch a file in the profile directory to indicate we're alive
   var foStream = Cc["@mozilla.org/network/file-output-stream;1"]
@@ -407,10 +401,8 @@ function testListing(metadata, response)
                  src: "/tests/SimpleTest/TestRunner.js"}),
         SCRIPT({type: "text/javascript",
                  src: "/tests/SimpleTest/MozillaFileLogger.js"}),
-/*
         SCRIPT({type: "text/javascript",
                  src: "/tests/SimpleTest/cross-domain.js"}),
- */
         SCRIPT({type: "text/javascript",
                  src: "/tests/SimpleTest/quit.js"}),
         SCRIPT({type: "text/javascript",

testing/mochitest/tests/SimpleTest/quit.js

@@ -100,11 +100,9 @@ function goQuitApplication()
     return false;
   }
   
-/*
   // shut down cross-domain mapping (really reverts preference values)
-  if (crossDomain)
+  if (typeof crossDomain != "undefined")
     crossDomain.disable();
-*/
 
   const kAppStartup = '@mozilla.org/toolkit/app-startup;1';
   const kAppShell   = '@mozilla.org/appshell/appShellService;1';