--- a/testing/mochitest/browser-test.js
+++ b/testing/mochitest/browser-test.js
@@ -164,16 +164,21 @@ Tester.prototype = {
         try {
           func.apply(testScope);
         }
         catch (ex) {
           this.currentTest.addResult(new testResult(false, "Cleanup function threw an exception", ex, false));
         }
       };
 
+      // Clear document.popupNode.  The test could have set it to a custom value
+      // for its own purposes, nulling it out it will go back to the default
+      // behavior of returning the last opened popup.
+      document.popupNode = null;
+
       // Note the test run time
       let time = Date.now() - this.lastStartTime;
       this.dumper.dump("INFO TEST-END | " + this.currentTest.path + " | finished in " + time + "ms\n");
       this.currentTest.setDuration(time);
     }
 
     // Check the window state for the current test before moving to the next one.
     // This also causes us to check before starting any tests, since nextTest()

--- a/testing/mochitest/tests/browser/Makefile.in
+++ b/testing/mochitest/tests/browser/Makefile.in
@@ -45,16 +45,18 @@ include $(DEPTH)/config/autoconf.mk
 include $(topsrcdir)/config/rules.mk
 
 _BROWSER_TEST_FILES = \
 	                  head.js \
 	                  browser_head.js \
 	                  browser_pass.js \
 	                  browser_async.js \
 	                  browser_privileges.js \
+	                  browser_popupNode.js \
+	                  browser_popupNode_check.js \
 # Disabled, these are only good for testing the harness' failure reporting
 #	                  browser_zz_fail_openwindow.js \
 #	                  browser_fail.js \
 #	                  browser_fail_async_throw.js \
 #	                  browser_fail_fp.js \
 #	                  browser_fail_pf.js \
 #	                  browser_fail_throw.js \
 #	                  browser_fail_timeout.js \

new file mode 100644
--- /dev/null
+++ b/testing/mochitest/tests/browser/browser_popupNode.js
@@ -0,0 +1,4 @@
+function test() {
+  document.popupNode = document;
+  isnot(document.popupNode, null, "document.popupNode has been correctly set");
+}

new file mode 100644
--- /dev/null
+++ b/testing/mochitest/tests/browser/browser_popupNode_check.js
@@ -0,0 +1,3 @@
+function test() {
+  is(document.popupNode, null, "document.popupNode has been correctly cleared");
+}