--- a/testing/marionette/interaction.js
+++ b/testing/marionette/interaction.js
@@ -408,31 +408,37 @@ interaction.flushEventLoop = async funct
     el.removeEventListener("click", clickEv);
   };
 
   return new TimedPromise(spinEventLoop, {timeout: 500, throws: null})
       .then(removeListeners);
 };
 
 /**
- * Focus element and, if a textual input field and no previous selection
- * state exists, move the caret to the end of the input field.
+ * If <var>el<var> is a textual form control and no previous
+ * selection state exists, move the caret to the end of the form control.
  *
- * @param {Element} element
- *     Element to focus.
+ * The element has to be a <code>&lt;input type=text&gt;</code>
+ * or <code>&lt;textarea&gt;</code> element for the cursor to move
+ * be moved.
+ *
+ * @param {Element} el
+ *     Element to potential move the caret in.
  */
-interaction.focusElement = function(el) {
-  let t = el.type;
-  if (t && (t == "text" || t == "textarea")) {
-    if (el.selectionEnd == 0) {
-      let len = el.value.length;
-      el.setSelectionRange(len, len);
-    }
+interaction.moveCaretToEnd = function(el) {
+  if (!element.isDOMElement(el) ||
+      el.localName != "textarea" ||
+      (el.localName != "input" && el.type == "text")) {
+    return;
   }
-  el.focus();
+
+  if (el.selectionEnd == 0) {
+    let len = el.value.length;
+    el.setSelectionRange(len, len);
+  }
 };
 
 /**
  * Performs checks if <var>el</var> is keyboard-interactable.
  *
  * To decide if an element is keyboard-interactable various properties,
  * and computed CSS styles have to be evaluated. Whereby it has to be taken
  * into account that the element can be part of a container (eg. option),
@@ -448,17 +454,16 @@ interaction.isKeyboardInteractable = fun
   const win = getWindow(el);
 
   // body and document element are always keyboard-interactable
   if (el.localName === "body" || el === win.document.documentElement) {
     return true;
   }
 
   el.focus();
-
   return el === win.document.activeElement;
 };
 
 /**
  * Appends <var>path</var> to an <tt>&lt;input type=file&gt;</tt>'s
  * file list.
  *
  * @param {HTMLInputElement} el
@@ -553,17 +558,18 @@ async function webdriverSendKeysToElemen
   if (!interaction.isKeyboardInteractable(containerEl)) {
     throw new ElementNotInteractableError(
         pprint`Element ${el} is not reachable by keyboard`);
   }
 
   let acc = await a11y.getAccessible(el, true);
   a11y.assertActionable(acc, el);
 
-  interaction.focusElement(el);
+  interaction.moveCaretToEnd(el);
+  el.focus();
 
   if (el.type == "file") {
     await interaction.uploadFile(el, value);
   } else if ((el.type == "date" || el.type == "time") &&
       Preferences.get("dom.forms.datetime")) {
     interaction.setFormControlValue(el, value);
   } else {
     event.sendKeysToElement(value, el, win);
@@ -586,17 +592,18 @@ async function legacySendKeysToElement(e
 
     if (!element.isVisible(visibilityCheckEl)) {
       throw new ElementNotInteractableError("Element is not visible");
     }
 
     let acc = await a11y.getAccessible(el, true);
     a11y.assertActionable(acc, el);
 
-    interaction.focusElement(el);
+    interaction.moveCaretToEnd(el);
+    el.focus();
     event.sendKeysToElement(value, el, win);
   }
 }
 
 /**
  * Determine the element displayedness of an element.
  *
  * @param {DOMElement|XULElement} el