--- a/testing/marionette/element.js
+++ b/testing/marionette/element.js
@@ -794,16 +794,52 @@ element.isSelected = function(el) {
  *     True if element is read only.
  */
 element.isReadOnly = function(el) {
   return element.isDOMElement(el) &&
       ["input", "textarea"].includes(el.localName) && el.readOnly;
 };
 
 /**
+ * An element is considered disabled if it is a an element
+ * that can be disabled, or it belongs to a container group which
+ * <code>disabled</code> content IDL attribute affects it.
+ *
+ * @param {Element} el
+ *     Element to test for disabledness.
+ *
+ * @return {boolean}
+ *     True if element, or its container group, is disabled.
+ */
+element.isDisabled = function(el) {
+  if (!element.isDOMElement(el)) {
+    return false;
+  }
+
+  switch (el.localName) {
+    case "option":
+    case "optgroup":
+      if (el.disabled) {
+        return true;
+      }
+      let parent = element.findClosest(el, "optgroup,select");
+      return element.isDisabled(parent);
+
+    case "button":
+    case "input":
+    case "select":
+    case "textarea":
+      return el.disabled;
+
+    default:
+      return false;
+  }
+};
+
+/**
  * This function generates a pair of coordinates relative to the viewport
  * given a target element and coordinates relative to that element's
  * top-left corner.
  *
  * @param {Node} node
  *     Target node.
  * @param {number=} xOffset
  *     Horizontal offset relative to target's top-left corner.

--- a/testing/marionette/test_element.js
+++ b/testing/marionette/test_element.js
@@ -199,16 +199,40 @@ add_test(function test_isReadOnly() {
   ok(!element.isReadOnly(domEl));
   ok(!element.isReadOnly(new DOMElement("p", {readOnly: true})));
   ok(element.isReadOnly(new DOMElement("input", {readOnly: true})));
   ok(element.isReadOnly(new DOMElement("textarea", {readOnly: true})));
 
   run_next_test();
 });
 
+add_test(function test_isDisabled() {
+  ok(!element.isDisabled(new DOMElement("p")));
+  ok(!element.isDisabled(new SVGElement("rect", {disabled: true})));
+  ok(!element.isDisabled(new XULElement("browser", {disabled: true})));
+
+  let select = new DOMElement("select", {disabled: true});
+  let option = new DOMElement("option");
+  option.parentNode = select;
+  ok(element.isDisabled(option));
+
+  let optgroup = new DOMElement("optgroup", {disabled: true});
+  option.parentNode = optgroup;
+  optgroup.parentNode = select;
+  select.disabled = false;
+  ok(element.isDisabled(option));
+
+  ok(element.isDisabled(new DOMElement("button", {disabled: true})));
+  ok(element.isDisabled(new DOMElement("input", {disabled: true})));
+  ok(element.isDisabled(new DOMElement("select", {disabled: true})));
+  ok(element.isDisabled(new DOMElement("textarea", {disabled: true})));
+
+  run_next_test();
+});
+
 add_test(function test_coordinates() {
   let p = element.coordinates(domEl);
   ok(p.hasOwnProperty("x"));
   ok(p.hasOwnProperty("y"));
   equal("number", typeof p.x);
   equal("number", typeof p.y);
 
   deepEqual({x: 50, y: 50}, element.coordinates(domEl));