diff --git a/dom/browser-element/BrowserElementChildPreload.js b/dom/browser-element/BrowserElementChildPreload.js
index 75f62604e603..e6bbf89733ee 100644
--- a/dom/browser-element/BrowserElementChildPreload.js
+++ b/dom/browser-element/BrowserElementChildPreload.js
@@ -379,6 +379,10 @@ BrowserElementChild.prototype = {
     }
     debug("Nested event loop - finish");
 
+    if (win.modalDepth == 0) {
+      delete this._windowIDDict[outerWindowID];
+    }
+
     // If we exited the loop because the inner window changed, then bail on the
     // modal prompt.
     if (innerWindowID !== this._tryGetInnerWindowID(win)) {
@@ -411,7 +415,6 @@ BrowserElementChild.prototype = {
     }
 
     let win = this._windowIDDict[outerID].get();
-    delete this._windowIDDict[outerID];
 
     if (!win) {
       debug("recvStopWaiting, but window is gone\n");
diff --git a/dom/browser-element/mochitest/browserElement_Alert.js b/dom/browser-element/mochitest/browserElement_Alert.js
index 0c63ab39661f..269996bb5174 100644
--- a/dom/browser-element/mochitest/browserElement_Alert.js
+++ b/dom/browser-element/mochitest/browserElement_Alert.js
@@ -124,7 +124,7 @@ function test4() {
 
 // test4 is a mozbrowsershowmodalprompt listener.
 function test5(e) {
-  iframe.removeEventListener('mozbrowsershowmodalprompt', test4);
+  iframe.removeEventListener('mozbrowsershowmodalprompt', test5);
 
   is(e.detail.message, 'test4');
   e.preventDefault(); // cause the page to block.
@@ -139,6 +139,140 @@ function test5a() {
 
 function test5b() {
   iframe.removeEventListener('mozbrowserloadend', test5b);
+  SimpleTest.executeSoon(test6);
+}
+
+// Test nested alerts
+var promptBlockers = [];
+function test6() {
+  iframe.addEventListener("mozbrowsershowmodalprompt", test6a);
+
+  var script = 'data:,\
+    this.testState = 0; \
+    content.alert(1); \
+    this.testState = 3; \
+  ';
+  mm.loadFrameScript(script, /* allowDelayedLoad = */ false);
+}
+
+function test6a(e) {
+  iframe.removeEventListener("mozbrowsershowmodalprompt", test6a);
+
+  is(e.detail.message, '1');
+  e.preventDefault(); // cause the alert to block.
+  promptBlockers.push(e);
+
+  SimpleTest.executeSoon(test6b);
+}
+
+function test6b() {
+  var script = 'data:,\
+    if (this.testState === 0) { \
+      sendAsyncMessage("test-success", "1: Correct testState"); \
+    } \
+    else { \
+      sendAsyncMessage("test-fail", "1: Wrong testState: " + this.testState); \
+    }';
+  mm.loadFrameScript(script, /* allowDelayedLoad = */ false);
+  numPendingChildTests++;
+
+  waitForPendingTests(test6c);
+}
+
+function test6c() {
+  iframe.addEventListener("mozbrowsershowmodalprompt", test6d);
+
+  var script = 'data:,\
+    this.testState = 1; \
+    content.alert(2); \
+    this.testState = 2; \
+  ';
+  mm.loadFrameScript(script, /* allowDelayedLoad = */ false);
+}
+
+function test6d(e) {
+  iframe.removeEventListener("mozbrowsershowmodalprompt", test6d);
+
+  is(e.detail.message, '2');
+  e.preventDefault(); // cause the alert to block.
+  promptBlockers.push(e);
+
+  SimpleTest.executeSoon(test6e);
+}
+
+function test6e() {
+  var script = 'data:,\
+    if (this.testState === 1) { \
+      sendAsyncMessage("test-success", "2: Correct testState"); \
+    } \
+    else { \
+      sendAsyncMessage("test-fail", "2: Wrong testState: " + this.testState); \
+    }';
+  mm.loadFrameScript(script, /* allowDelayedLoad = */ false);
+  numPendingChildTests++;
+
+  waitForPendingTests(test6f);
+}
+
+function test6f() {
+  var e = promptBlockers.pop();
+  // Now unblock the iframe and check that the script completed.
+  e.detail.unblock();
+
+  var script2 = 'data:,\
+    if (this.testState === 2) { \
+      sendAsyncMessage("test-success", "3: Correct testState"); \
+    } \
+    else { \
+      sendAsyncMessage("test-try-again", "3: Wrong testState (for now): " + this.testState); \
+    }';
+
+  // Urgh.  e.unblock() didn't necessarily unblock us immediately, so we have
+  // to spin and wait.
+  function onTryAgain() {
+    SimpleTest.executeSoon(function() {
+      //dump('onTryAgain\n');
+      mm.loadFrameScript(script2, /* allowDelayedLoad = */ false);
+    });
+  }
+
+  mm.addMessageListener('test-try-again', onTryAgain);
+  numPendingChildTests++;
+
+  onTryAgain();
+  waitForPendingTests(test6g);
+}
+
+function test6g() {
+  var e = promptBlockers.pop();
+  // Now unblock the iframe and check that the script completed.
+  e.detail.unblock();
+
+  var script2 = 'data:,\
+    if (this.testState === 3) { \
+      sendAsyncMessage("test-success", "4: Correct testState"); \
+    } \
+    else { \
+      sendAsyncMessage("test-try-again", "4: Wrong testState (for now): " + this.testState); \
+    }';
+
+  // Urgh.  e.unblock() didn't necessarily unblock us immediately, so we have
+  // to spin and wait.
+  function onTryAgain() {
+    SimpleTest.executeSoon(function() {
+      //dump('onTryAgain\n');
+      mm.loadFrameScript(script2, /* allowDelayedLoad = */ false);
+    });
+  }
+
+  mm.addMessageListener('test-try-again', onTryAgain);
+  numPendingChildTests++;
+
+  onTryAgain();
+  waitForPendingTests(test6h);
+}
+
+function test6h() {
   SimpleTest.finish();
 }