diff --git a/dom/events/ContentEventHandler.cpp b/dom/events/ContentEventHandler.cpp
index 2e86d7e7e73..4556b107bc7 100644
--- a/dom/events/ContentEventHandler.cpp
+++ b/dom/events/ContentEventHandler.cpp
@@ -267,7 +267,7 @@ static uint32_t CountNewlinesInNativeLength(nsIContent* aContent,
     return 0;
   }
   // For automated tests, we should abort on debug build.
-  NS_ABORT_IF_FALSE(
+  MOZ_ASSERT(
     (aNativeLength == UINT32_MAX || aNativeLength <= text->GetLength() * 2),
     "aNativeLength is unexpected value");
   const uint32_t xpLength = text->GetLength();
diff --git a/editor/libeditor/html/tests/mochitest.ini b/editor/libeditor/html/tests/mochitest.ini
index b8ed9aab3a9..2ff15ae6883 100644
--- a/editor/libeditor/html/tests/mochitest.ini
+++ b/editor/libeditor/html/tests/mochitest.ini
@@ -98,6 +98,7 @@ skip-if = toolkit == 'android' || e10s
 skip-if = os != "win"
 [test_bug966552.html]
 skip-if = os != "win"
+[test_bug998188.html]
 [test_contenteditable_focus.html]
 [test_dom_input_event_on_htmleditor.html]
 [test_keypress_untrusted_event.html]
diff --git a/editor/libeditor/html/tests/test_bug998188.html b/editor/libeditor/html/tests/test_bug998188.html
new file mode 100644
index 00000000000..2d167f0bd6e
--- /dev/null
+++ b/editor/libeditor/html/tests/test_bug998188.html
@@ -0,0 +1,52 @@
+<!DOCTYPE HTML>
+<html>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=565392
+-->
+<head>
+  <title>Test for Bug 998188</title>
+  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script type="application/javascript" src="/tests/SimpleTest/EventUtils.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
+</head>
+<body>
+<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=998188">Mozilla Bug 998188</a>
+<p id="display"></p>
+<div id="content" style="display: none">
+  
+</div>
+<div id="editor" contenteditable>abc</div>
+<pre id="test">
+<script type="application/javascript">
+
+/** Test for Bug 998188 **/
+
+SimpleTest.waitForExplicitFinish();
+
+function runTests()
+{
+  var editor = document.getElementById("editor");
+  editor.focus();
+
+  var textNode1 = document.createTextNode("def");
+  var textNode2 = document.createTextNode("ghi");
+
+  editor.appendChild(textNode1);
+  editor.appendChild(textNode2);
+
+  window.getSelection().collapse(textNode2, 3);
+
+  for (var i = 0; i < 9; i++) {
+    var caretRect = synthesizeQueryCaretRect(i);
+    ok(caretRect.succeeded, "QueryCaretRect should succeeded (" + i + ")");
+  }
+
+  SimpleTest.finish();
+}
+
+SimpleTest.waitForFocus(runTests);
+
+</script>
+</pre>
+</body>
+</html>
diff --git a/testing/mochitest/tests/SimpleTest/ChromeUtils.js b/testing/mochitest/tests/SimpleTest/ChromeUtils.js
index 0b4085493b7..ac2a376679f 100644
--- a/testing/mochitest/tests/SimpleTest/ChromeUtils.js
+++ b/testing/mochitest/tests/SimpleTest/ChromeUtils.js
@@ -33,25 +33,6 @@ function synthesizeQueryTextContent(aOffset, aLength, aWindow)
                                      aOffset, aLength, 0, 0);
 }
 
-/**
- * Synthesize a query caret rect event.
- *
- * @param aOffset  The caret offset.  0 means left side of the first character
- *                 in the selection root.
- * @param aWindow  Optional (If null, current |window| will be used)
- * @return         An nsIQueryContentEventResult object.  If this failed,
- *                 the result might be null.
- */
-function synthesizeQueryCaretRect(aOffset, aWindow)
-{
-  var utils = _getDOMWindowUtils(aWindow);
-  if (!utils) {
-    return nullptr;
-  }
-  return utils.sendQueryContentEvent(utils.QUERY_CARET_RECT,
-                                     aOffset, 0, 0, 0);
-}
-
 /**
  * Synthesize a query text rect event.
  *
diff --git a/testing/mochitest/tests/SimpleTest/EventUtils.js b/testing/mochitest/tests/SimpleTest/EventUtils.js
index 932eb72b2e3..92437b3caab 100644
--- a/testing/mochitest/tests/SimpleTest/EventUtils.js
+++ b/testing/mochitest/tests/SimpleTest/EventUtils.js
@@ -988,6 +988,25 @@ function synthesizeQuerySelectedText(aWindow)
   return utils.sendQueryContentEvent(utils.QUERY_SELECTED_TEXT, 0, 0, 0, 0);
 }
 
+/**
+ * Synthesize a query caret rect event.
+ *
+ * @param aOffset  The caret offset.  0 means left side of the first character
+ *                 in the selection root.
+ * @param aWindow  Optional (If null, current |window| will be used)
+ * @return         An nsIQueryContentEventResult object.  If this failed,
+ *                 the result might be null.
+ */
+function synthesizeQueryCaretRect(aOffset, aWindow)
+{
+  var utils = _getDOMWindowUtils(aWindow);
+  if (!utils) {
+    return null;
+  }
+  return utils.sendQueryContentEvent(utils.QUERY_CARET_RECT,
+                                     aOffset, 0, 0, 0);
+}
+
 /**
  * Synthesize a selection set event.
  *
