--- a/testing/marionette/element.js
+++ b/testing/marionette/element.js
@@ -291,57 +291,58 @@ function find_(container, strategy, sele
     return res;
   } else if (res) {
     return [res];
   }
   return [];
 }
 
 /**
- * Find a value by XPATH
+ * Find a single element by XPath expression.
+ *
+ * @param {DOMElement} root
+ *     Document root
+ * @param {DOMElement} startNode
+ *     Where in the DOM hiearchy to begin searching.
+ * @param {string} expr
+ *     XPath search expression.
  *
- * @param nsIDOMElement root
- *        Document root
- * @param string value
- *        XPATH search string
- * @param nsIDOMElement node
- *        start node
- *
- * @return nsIDOMElement
- *        returns the found element
+ * @return {DOMElement}
+ *     First element matching expression.
  */
-function findByXPath(root, value, node) {
-  return root.evaluate(value, node, null,
-          Ci.nsIDOMXPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;
-}
+element.findByXPath = function(root, startNode, expr) {
+  let iter = root.evaluate(expr, startNode, null,
+      Ci.nsIDOMXPathResult.FIRST_ORDERED_NODE_TYPE, null)
+  return iter.singleNodeValue;
+};
 
 /**
- * Find values by XPATH
+ * Find elements by XPath expression.
  *
- * @param nsIDOMElement root
- *        Document root
- * @param string value
- *        XPATH search string
- * @param nsIDOMElement node
- *        start node
+ * @param {DOMElement} root
+ *     Document root.
+ * @param {DOMElement} startNode
+ *     Where in the DOM hierarchy to begin searching.
+ * @param {string} expr
+ *     XPath search expression.
  *
- * @return object
- *        returns a list of found nsIDOMElements
+ * @return {Array.<DOMElement>}
+ *     Sequence of found elements matching expression.
  */
-function findByXPathAll(root, value, node) {
-  let values = root.evaluate(value, node, null,
-                    Ci.nsIDOMXPathResult.ORDERED_NODE_ITERATOR_TYPE, null);
-  let elements = [];
-  let element = values.iterateNext();
-  while (element) {
-    elements.push(element);
-    element = values.iterateNext();
+element.findByXPathAll = function(root, startNode, expr) {
+  let rv = [];
+  let iter = root.evaluate(expr, startNode, null,
+      Ci.nsIDOMXPathResult.ORDERED_NODE_ITERATOR_TYPE, null);
+  let el = iter.iterateNext();
+  while (el) {
+    rv.push(el);
+    el = iter.iterateNext();
   }
-  return elements;
-}
+  return rv;
+};
 
 /**
  * Find all hyperlinks dscendant of |node| which link text is |s|.
  *
  * @param {DOMElement} node
  *     Where in the DOM hierarchy to being searching.
  * @param {string} s
  *     Link text to search for.
@@ -411,34 +412,34 @@ function filterLinks(node, predicate) {
  *     If selector expression |value| is malformed.
  */
 function findElement(using, value, rootNode, startNode) {
   switch (using) {
     case element.Strategy.ID:
       if (startNode.getElementById) {
         return startNode.getElementById(value);
       }
-      return findByXPath(rootNode, `.//*[@id="${value}"]`, startNode);
+      return element.findByXPath(rootNode, startNode, `.//*[@id="${value}"]`);
 
     case element.Strategy.Name:
       if (startNode.getElementsByName) {
         return startNode.getElementsByName(value)[0];
       }
-      return findByXPath(rootNode, `.//*[@name="${value}"]`, startNode);
+      return element.findByXPath(rootNode, startNode, `.//*[@name="${value}"]`);
 
     case element.Strategy.ClassName:
       // works for >= Firefox 3
       return  startNode.getElementsByClassName(value)[0];
 
     case element.Strategy.TagName:
       // works for all elements
       return startNode.getElementsByTagName(value)[0];
 
     case element.Strategy.XPath:
-      return  findByXPath(rootNode, value, startNode);
+      return  element.findByXPath(rootNode, startNode, value);
 
     case element.Strategy.LinkText:
       for (let link of startNode.getElementsByTagName("a")) {
         if (link.text === value) {
           return link;
         }
       }
       break;
@@ -492,23 +493,23 @@ function findElement(using, value, rootN
  */
 function findElements(using, value, rootNode, startNode) {
   switch (using) {
     case element.Strategy.ID:
       value = `.//*[@id="${value}"]`;
 
     // fall through
     case element.Strategy.XPath:
-      return findByXPathAll(rootNode, value, startNode);
+      return element.findByXPathAll(rootNode, startNode, value);
 
     case element.Strategy.Name:
       if (startNode.getElementsByName) {
         return startNode.getElementsByName(value);
       }
-      return findByXPathAll(rootNode, `.//*[@name="${value}"]`, startNode);
+      return element.findByXPathAll(rootNode, startNode, `.//*[@name="${value}"]`);
 
     case element.Strategy.ClassName:
       return startNode.getElementsByClassName(value);
 
     case element.Strategy.TagName:
       return startNode.getElementsByTagName(value);
 
     case element.Strategy.LinkText: