diff --git a/testing/mochitest/browser-harness.xul b/testing/mochitest/browser-harness.xul
index 7ad1a3f7bd4..1e61665f3c5 100644
--- a/testing/mochitest/browser-harness.xul
+++ b/testing/mochitest/browser-harness.xul
@@ -117,6 +117,8 @@
       this.results = [];
       this.scope = null;
       this.duration = 0;
+      this.unexpectedTimeouts = 0;
+      this.lastOutputTime = 0;
     }
     browserTest.prototype = {
       get passCount() {
@@ -130,6 +132,7 @@
       },
 
       addResult: function addResult(result) {
+        this.lastOutputTime = Date.now();
         this.results.push(result);
 
         this.dumper.dump(result.result + " | " + this.path + " | " + result.msg + "\n");
diff --git a/testing/mochitest/browser-test.js b/testing/mochitest/browser-test.js
index 59600a159fe..4495ddc56c2 100644
--- a/testing/mochitest/browser-test.js
+++ b/testing/mochitest/browser-test.js
@@ -40,7 +40,7 @@ function testOnLoad() {
                            "chrome,centerscreen,dialog=no,resizable,titlebar,toolbar=no,width=800,height=600", sstring);
   } else {
     // This code allows us to redirect without requiring specialpowers for chrome and a11y tests.
-    function messageHandler(m) {
+    let messageHandler = function(m) {
       messageManager.removeMessageListener("chromeEvent", messageHandler);
       var url = m.json.data;
 
@@ -49,7 +49,7 @@ function testOnLoad() {
       var webNav = content.window.QueryInterface(Components.interfaces.nsIInterfaceRequestor)
                          .getInterface(Components.interfaces.nsIWebNavigation);
       webNav.loadURI(url, null, null, null, null);
-    }
+    };
 
     var listener = 'data:,function doLoad(e) { var data=e.getData("data");removeEventListener("contentEvent", function (e) { doLoad(e); }, false, true);sendAsyncMessage("chromeEvent", {"data":data}); };addEventListener("contentEvent", function (e) { doLoad(e); }, false, true);';
     messageManager.loadFrameScript(listener, true);
@@ -303,6 +303,14 @@ Tester.prototype = {
       // behavior of returning the last opened popup.
       document.popupNode = null;
 
+      // Notify a long running test problem if it didn't end up in a timeout.
+      if (this.currentTest.unexpectedTimeouts && !this.currentTest.timedOut) {
+        let msg = "This test exceeded the timeout threshold. It should be " +
+                  "rewritten or split up. If that's not possible, use " +
+                  "requestLongerTimeout(N), but only as a last resort.";
+        this.currentTest.addResult(new testResult(false, msg, "", false));
+      }
+
       // 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");
@@ -453,16 +461,32 @@ Tester.prototype = {
     }
     else {
       var self = this;
-      this.currentTest.scope.__waitTimer = setTimeout(function() {
+      this.currentTest.scope.__waitTimer = setTimeout(function timeoutFn() {
         if (--self.currentTest.scope.__timeoutFactor > 0) {
           // We were asked to wait a bit longer.
           self.currentTest.scope.info(
             "Longer timeout required, waiting longer...  Remaining timeouts: " +
             self.currentTest.scope.__timeoutFactor);
           self.currentTest.scope.__waitTimer =
-            setTimeout(arguments.callee, gTimeoutSeconds * 1000);
+            setTimeout(timeoutFn, gTimeoutSeconds * 1000);
           return;
         }
+
+        // If the test is taking longer than expected, but it's not hanging,
+        // mark the fact, but let the test continue.  At the end of the test,
+        // if it didn't timeout, we will notify the problem through an error.
+        // To figure whether it's an actual hang, compare the time of the last
+        // result or message to half of the timeout time.
+        // Though, to protect against infinite loops, limit the number of times
+        // we allow the test to proceed.
+        const MAX_UNEXPECTED_TIMEOUTS = 10;
+        if (Date.now() - self.currentTest.lastOutputTime < (gTimeoutSeconds / 2) * 1000 &&
+            ++self.currentTest.unexpectedTimeouts <= MAX_UNEXPECTED_TIMEOUTS) {
+            self.currentTest.scope.__waitTimer =
+              setTimeout(timeoutFn, gTimeoutSeconds * 1000);
+          return;
+        }
+
         self.currentTest.addResult(new testResult(false, "Test timed out", "", false));
         self.currentTest.timedOut = true;
         self.currentTest.scope.__waitTimer = null;
diff --git a/testing/mochitest/tests/browser/Makefile.in b/testing/mochitest/tests/browser/Makefile.in
index 1376948e72f..9749d49623d 100644
--- a/testing/mochitest/tests/browser/Makefile.in
+++ b/testing/mochitest/tests/browser/Makefile.in
@@ -11,25 +11,28 @@ relativesrcdir  = @relativesrcdir@
 include $(DEPTH)/config/autoconf.mk
 
 MOCHITEST_BROWSER_FILES = \
-	                  head.js \
-	                  browser_head.js \
-	                  browser_pass.js \
-	                  browser_async.js \
-	                  browser_privileges.js \
-	                  browser_popupNode.js \
-	                  browser_popupNode_check.js \
-			  browser_sanityException.js \
-			  browser_sanityException2.js \
-			  $(NULL)
+  head.js \
+  browser_head.js \
+  browser_pass.js \
+  browser_async.js \
+  browser_privileges.js \
+  browser_popupNode.js \
+  browser_popupNode_check.js \
+  browser_sanityException.js \
+  browser_sanityException2.js \
+  $(NULL)
+
 # 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 \
+#  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 \
+#  browser_fail_unexpectedTimeout.js \
+#
 # Disabled because it would take too long, useful to check functionality though.
-#	                  browser_requestLongerTimeout.js \
+#  browser_requestLongerTimeout.js \
 
 include $(topsrcdir)/config/rules.mk
diff --git a/testing/mochitest/tests/browser/browser_fail_timeout.js b/testing/mochitest/tests/browser/browser_fail_timeout.js
index 4ba6a519e5a..6b99693d972 100644
--- a/testing/mochitest/tests/browser/browser_fail_timeout.js
+++ b/testing/mochitest/tests/browser/browser_fail_timeout.js
@@ -1,6 +1,6 @@
 function test() {
   function end() {
-    ok(true, "didn't time out?");
+    ok(false, "should have timed out");
     finish();
   }
   waitForExplicitFinish();
diff --git a/testing/mochitest/tests/browser/browser_fail_unexpectedTimeout.js b/testing/mochitest/tests/browser/browser_fail_unexpectedTimeout.js
new file mode 100644
index 00000000000..2175eea2733
--- /dev/null
+++ b/testing/mochitest/tests/browser/browser_fail_unexpectedTimeout.js
@@ -0,0 +1,12 @@
+function test() {
+  function message() {
+    info("This should delay timeout");
+  }
+  function end() {
+    ok(true, "Should have not timed out, but notified long running test");
+    finish();
+  }
+  waitForExplicitFinish();
+  setTimeout(message, 20000);
+  setTimeout(end, 40000);
+}