diff --git a/docshell/test/browser/Makefile.in b/docshell/test/browser/Makefile.in
index 99fb3c4d00d1..6b407998ad9a 100644
--- a/docshell/test/browser/Makefile.in
+++ b/docshell/test/browser/Makefile.in
@@ -45,6 +45,8 @@ include $(topsrcdir)/config/rules.mk
 
 _BROWSER_TEST_FILES =	\
 		browser_bug349769.js \
+		browser_bug388121-1.js \
+		browser_bug388121-2.js \
 		$(NULL)
 
 libs:: $(_BROWSER_TEST_FILES)
diff --git a/docshell/test/browser/browser_bug349769.js b/docshell/test/browser/browser_bug349769.js
index 14a7536d8bdd..eecf6bf892ec 100644
--- a/docshell/test/browser/browser_bug349769.js
+++ b/docshell/test/browser/browser_bug349769.js
@@ -11,6 +11,7 @@ function test() {
 
   function testLoad(event) {
     newBrowser.removeEventListener("load", testLoad, true);
+    is (event.target, newBrowser.contentDocument, "Unexpected target");
     var prin = newBrowser.contentDocument.nodePrincipal;
     isnot(prin, null, "Loaded principal must not be null when adding " + uri);
     isnot(prin, undefined, "Loaded principal must not be undefined when loading " + uri);
diff --git a/docshell/test/browser/browser_bug388121-1.js b/docshell/test/browser/browser_bug388121-1.js
new file mode 100644
index 000000000000..54c3140e9878
--- /dev/null
+++ b/docshell/test/browser/browser_bug388121-1.js
@@ -0,0 +1,27 @@
+function test() {
+  waitForExplicitFinish(); 
+
+  var newTab;
+  var newBrowser;
+  const secMan = Components.classes["@mozilla.org/scriptsecuritymanager;1"].
+                   getService(Components.interfaces.nsIScriptSecurityManager);
+
+  function testLoad(event) {
+    newBrowser.removeEventListener("load", testLoad, true);
+    is (event.target, newBrowser.contentDocument, "Unexpected target");
+    var prin = newBrowser.contentDocument.nodePrincipal;
+    isnot(prin, null, "Loaded principal must not be null");
+    isnot(prin, undefined, "Loaded principal must not be undefined");
+    is(secMan.isSystemPrincipal(prin), false,
+       "Loaded principal must not be system");
+    gBrowser.removeTab(newTab);
+
+    finish();
+  }
+
+  newTab = gBrowser.addTab();
+  newBrowser = gBrowser.getBrowserForTab(newTab);
+  newBrowser.contentWindow.location.href = "about:blank"
+  newBrowser.addEventListener("load", testLoad, true);
+}
+
diff --git a/docshell/test/browser/browser_bug388121-2.js b/docshell/test/browser/browser_bug388121-2.js
new file mode 100644
index 000000000000..5f5d8c422246
--- /dev/null
+++ b/docshell/test/browser/browser_bug388121-2.js
@@ -0,0 +1,59 @@
+function test() {
+  waitForExplicitFinish(); 
+
+  var w;
+  const secMan = Components.classes["@mozilla.org/scriptsecuritymanager;1"].
+                   getService(Components.interfaces.nsIScriptSecurityManager);
+  var iteration = 1;
+  const uris = ["", "about:blank"];
+  var uri;
+  var origDoc;
+
+  function testLoad() {
+    if (w.document == origDoc) {
+      // Go back to polling
+      setTimeout(testLoad, 10);
+      return;
+    }
+    var prin = w.document.nodePrincipal;
+    isnot(prin, null, "Loaded principal must not be null when adding " + uri);
+    isnot(prin, undefined, "Loaded principal must not be undefined when loading " + uri);
+    is(secMan.isSystemPrincipal(prin), false,
+       "Loaded principal must not be system when loading " + uri);
+    w.close();
+
+    if (iteration == uris.length) {
+      finish();
+    } else {
+      ++iteration;
+      doTest();
+    }
+  }
+
+  function doTest() {
+    uri = uris[iteration - 1];
+    w = window.open(uri, "_blank", "width=10,height=10");
+    var prin = w.document.nodePrincipal;
+    if (!uri) {
+      uri = undefined;
+    }
+    isnot(prin, null, "Forced principal must not be null when loading " + uri);
+    isnot(prin, undefined,
+          "Forced principal must not be undefined when loading " + uri);
+    is(secMan.isSystemPrincipal(prin), false,
+       "Forced principal must not be system when loading " + uri);
+    if (uri == undefined) {
+      // No actual load here, so just move along.
+      w.close();
+      ++iteration;
+      doTest();
+    } else {
+      origDoc = w.document;
+      // Need to poll, because load listeners on the content window won't
+      // survive the load.
+      setTimeout(testLoad, 10);
+    }
+  }
+
+  doTest();
+}