--- a/testing/marionette/listener.js
+++ b/testing/marionette/listener.js
@@ -21,16 +21,17 @@ Cu.import("chrome://marionette/content/e
 Cu.import("chrome://marionette/content/error.js");
 Cu.import("chrome://marionette/content/evaluate.js");
 Cu.import("chrome://marionette/content/event.js");
 Cu.import("chrome://marionette/content/interaction.js");
 Cu.import("chrome://marionette/content/legacyaction.js");
 Cu.import("chrome://marionette/content/logging.js");
 Cu.import("chrome://marionette/content/navigate.js");
 Cu.import("chrome://marionette/content/proxy.js");
+Cu.import("chrome://marionette/content/session.js");
 Cu.import("chrome://marionette/content/simpletest.js");
 
 Cu.import("resource://gre/modules/FileUtils.jsm");
 Cu.import("resource://gre/modules/Task.jsm");
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 
 Cu.importGlobalProperties(["URL"]);
 
@@ -53,17 +54,17 @@ var SUPPORTED_STRATEGIES = new Set([
   element.Strategy.ID,
   element.Strategy.Name,
   element.Strategy.LinkText,
   element.Strategy.PartialLinkText,
   element.Strategy.TagName,
   element.Strategy.XPath,
 ]);
 
-var capabilities = {};
+var capabilities;
 
 var legacyactions = new legacyaction.Chain(checkForInterrupted);
 
 // the unload handler
 var onunload;
 
 // Flag to indicate whether an async script is currently running or not.
 var asyncTestRunning = false;
@@ -118,17 +119,17 @@ var sandboxName = "default";
  */
 function registerSelf() {
   let msg = {value: winUtil.outerWindowID};
   // register will have the ID and a boolean describing if this is the main process or not
   let register = sendSyncMessage("Marionette:register", msg);
 
   if (register[0]) {
     let {id, remotenessChange} = register[0][0];
-    capabilities = register[0][2];
+    capabilities = session.Capabilities.fromJSON(register[0][2]);
     listenerId = id;
     if (typeof id != "undefined") {
       // check if we're the main process
       if (register[0][1]) {
         addMessageListener("MarionetteMainListener:emitTouchEvent", emitTouchEventForIFrame);
       }
       startListeners();
       let rv = {};
@@ -316,18 +317,18 @@ function waitForReady() {
   }
 }
 
 /**
  * Called when we start a new session. It registers the
  * current environment, and resets all values
  */
 function newSession(msg) {
-  capabilities = msg.json;
-  isB2G = capabilities.platformName == "B2G";
+  capabilities = session.Capabilities.fromJSON(msg.json);
+  isB2G = capabilities.get("platformName") === "B2G";
   resetValues();
   if (isB2G) {
     readyStateTimer.initWithCallback(waitForReady, 100, Ci.nsITimer.TYPE_ONE_SHOT);
     // We have to set correct mouse event source to MOZ_SOURCE_TOUCH
     // to offer a way for event listeners to differentiate
     // events being the result of a physical mouse action.
     // This is especially important for the touch event shim,
     // in order to prevent creating touch event for these fake mouse events.
@@ -641,17 +642,17 @@ function emitTouchEvent(type, touch) {
 function singleTap(id, corx, cory) {
   let el = seenEls.get(id, curContainer);
   // after this block, the element will be scrolled into view
   let visible = element.isVisible(el, corx, cory);
   if (!visible) {
     throw new ElementNotVisibleError("Element is not currently visible and may not be manipulated");
   }
 
-  let a11y = accessibility.get(capabilities["moz:accessibilityChecks"]);
+  let a11y = accessibility.get(capabilities.get("moz:accessibilityChecks"));
   return a11y.getAccessible(el, true).then(acc => {
     a11y.assertVisible(acc, el, visible);
     a11y.assertActionable(acc, el);
     if (!curContainer.frame.document.createTouch) {
       legacyactions.mouseEventsOnly = true;
     }
     let c = element.coordinates(el, corx, cory);
     if (!legacyactions.mouseEventsOnly) {
@@ -1201,18 +1202,18 @@ function getActiveElement() {
  *
  * @param {WebElement} id
  *     Reference to the web element to click.
  */
 function clickElement(id) {
   let el = seenEls.get(id, curContainer);
   return interaction.clickElement(
       el,
-      !!capabilities["moz:accessibilityChecks"],
-      capabilities.specificationLevel >= 1);
+      capabilities.get("moz:accessibilityChecks"),
+      capabilities.get("specificationLevel") >= 1);
 }
 
 function getElementAttribute(id, name) {
   let el = seenEls.get(id, curContainer);
   if (element.isBooleanAttribute(el, name)) {
     if (el.hasAttribute(name)) {
       return "true";
     } else {
@@ -1260,17 +1261,17 @@ function getElementTagName(id) {
  * Determine the element displayedness of the given web element.
  *
  * Also performs additional accessibility checks if enabled by session
  * capability.
  */
 function isElementDisplayed(id) {
   let el = seenEls.get(id, curContainer);
   return interaction.isElementDisplayed(
-      el, capabilities["moz:accessibilityChecks"]);
+      el, capabilities.get("moz:accessibilityChecks"));
 }
 
 /**
  * Retrieves the computed value of the given CSS property of the given
  * web element.
  *
  * @param {String} id
  *     Web element reference.
@@ -1313,39 +1314,39 @@ function getElementRect(id) {
  *     Reference to web element.
  *
  * @return {boolean}
  *     True if enabled, false otherwise.
  */
 function isElementEnabled(id) {
   let el = seenEls.get(id, curContainer);
   return interaction.isElementEnabled(
-      el, capabilities["moz:accessibilityChecks"]);
+      el, capabilities.get("moz:accessibilityChecks"));
 }
 
 /**
  * Determines if the referenced element is selected or not.
  *
  * This operation only makes sense on input elements of the Checkbox-
  * and Radio Button states, or option elements.
  */
 function isElementSelected(id) {
   let el = seenEls.get(id, curContainer);
   return interaction.isElementSelected(
-      el, capabilities["moz:accessibilityChecks"]);
+      el, capabilities.get("moz:accessibilityChecks"));
 }
 
 function* sendKeysToElement(id, val) {
   let el = seenEls.get(id, curContainer);
   if (el.type == "file") {
     let path = val.join("");
     yield interaction.uploadFile(el, path);
   } else {
     yield interaction.sendKeysToElement(
-        el, val, false, capabilities["moz:accessibilityChecks"]);
+        el, val, false, capabilities.get("moz:accessibilityChecks"));
   }
 }
 
 /**
  * Clear the text of an element.
  */
 function clearElement(id) {
   try {