--- a/testing/marionette/actors/MarionetteFrameParent.jsm
+++ b/testing/marionette/actors/MarionetteFrameParent.jsm
@@ -91,23 +91,25 @@ class MarionetteFrameParent extends JSWi
 
   cleanUp() {
     elementIdCache.clear();
   }
 
   // Proxying methods for WebDriver commands
   // TODO: Maybe using a proxy class instead similar to proxy.js
 
-  clearElement(elem) {
-    return this.sendQuery("MarionetteFrameParent:clearElement", { elem });
+  clearElement(webEl) {
+    return this.sendQuery("MarionetteFrameParent:clearElement", {
+      elem: webEl,
+    });
   }
 
-  clickElement(elem, capabilities) {
+  clickElement(webEl, capabilities) {
     return this.sendQuery("MarionetteFrameParent:clickElement", {
-      elem,
+      elem: webEl,
       capabilities,
     });
   }
 
   async executeScript(script, args, opts) {
     return this.sendQuery("MarionetteFrameParent:executeScript", {
       script,
       args,
@@ -134,100 +136,106 @@ class MarionetteFrameParent extends JSWi
   async getActiveElement() {
     return this.sendQuery("MarionetteFrameParent:getActiveElement");
   }
 
   async getCurrentUrl() {
     return this.sendQuery("MarionetteFrameParent:getCurrentUrl");
   }
 
-  async getElementAttribute(elem, name) {
+  async getElementAttribute(webEl, name) {
     return this.sendQuery("MarionetteFrameParent:getElementAttribute", {
-      elem,
+      elem: webEl,
       name,
     });
   }
 
-  async getElementProperty(elem, name) {
+  async getElementProperty(webEl, name) {
     return this.sendQuery("MarionetteFrameParent:getElementProperty", {
-      elem,
+      elem: webEl,
       name,
     });
   }
 
-  async getElementRect(elem) {
-    return this.sendQuery("MarionetteFrameParent:getElementRect", { elem });
+  async getElementRect(webEl) {
+    return this.sendQuery("MarionetteFrameParent:getElementRect", {
+      elem: webEl,
+    });
   }
 
-  async getElementTagName(elem) {
-    return this.sendQuery("MarionetteFrameParent:getElementTagName", { elem });
+  async getElementTagName(webEl) {
+    return this.sendQuery("MarionetteFrameParent:getElementTagName", {
+      elem: webEl,
+    });
   }
 
-  async getElementText(elem) {
-    return this.sendQuery("MarionetteFrameParent:getElementText", { elem });
+  async getElementText(webEl) {
+    return this.sendQuery("MarionetteFrameParent:getElementText", {
+      elem: webEl,
+    });
   }
 
-  async getElementValueOfCssProperty(elem, name) {
+  async getElementValueOfCssProperty(webEl, name) {
     return this.sendQuery(
       "MarionetteFrameParent:getElementValueOfCssProperty",
       {
-        elem,
+        elem: webEl,
         name,
       }
     );
   }
 
   async getPageSource() {
     return this.sendQuery("MarionetteFrameParent:getPageSource");
   }
 
-  async isElementDisplayed(elem, capabilities) {
+  async isElementDisplayed(webEl, capabilities) {
     return this.sendQuery("MarionetteFrameParent:isElementDisplayed", {
       capabilities,
-      elem,
+      elem: webEl,
     });
   }
 
-  async isElementEnabled(elem, capabilities) {
+  async isElementEnabled(webEl, capabilities) {
     return this.sendQuery("MarionetteFrameParent:isElementEnabled", {
       capabilities,
-      elem,
+      elem: webEl,
     });
   }
 
-  async isElementSelected(elem, capabilities) {
+  async isElementSelected(webEl, capabilities) {
     return this.sendQuery("MarionetteFrameParent:isElementSelected", {
       capabilities,
-      elem,
+      elem: webEl,
     });
   }
 
-  async sendKeysToElement(elem, text, capabilities) {
+  async sendKeysToElement(webEl, text, capabilities) {
     return this.sendQuery("MarionetteFrameParent:sendKeysToElement", {
       capabilities,
-      elem,
+      elem: webEl,
       text,
     });
   }
 
   async performActions(actions, capabilities) {
     return this.sendQuery("MarionetteFrameParent:performActions", {
       actions,
       capabilities,
     });
   }
 
   async releaseActions() {
     return this.sendQuery("MarionetteFrameParent:releaseActions");
   }
 
-  async singleTap(elem, x, y, capabilities) {
+  async singleTap(webEl, x, y, capabilities) {
     return this.sendQuery("MarionetteFrameParent:singleTap", {
       capabilities,
-      elem,
+      elem: webEl,
       x,
       y,
     });
   }
 
   async switchToFrame(id) {
     const {
       browsingContextId,
@@ -243,29 +251,29 @@ class MarionetteFrameParent extends JSWi
       "MarionetteFrameParent:switchToParentFrame"
     );
 
     return {
       browsingContext: BrowsingContext.get(browsingContextId),
     };
   }
 
-  async takeScreenshot(elem, format, full, scroll) {
+  async takeScreenshot(webEl, format, full, scroll) {
     const rect = await this.sendQuery(
       "MarionetteFrameParent:getScreenshotRect",
       {
-        elem,
+        elem: webEl,
         full,
         scroll,
       }
     );
 
     // If no element has been specified use the top-level browsing context.
     // Otherwise use the browsing context from the currently selected frame.
-    const browsingContext = elem
+    const browsingContext = webEl
       ? this.browsingContext
       : this.browsingContext.top;
 
     let canvas = await capture.canvas(
       browsingContext.topChromeWindow,
       browsingContext,
       rect.x,
       rect.y,