--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -13,16 +13,17 @@ Cu.import("resource://gre/modules/Log.js
 Cu.import("resource://gre/modules/Preferences.jsm");
 Cu.import("resource://gre/modules/Services.jsm");
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 
 XPCOMUtils.defineLazyServiceGetter(
     this, "cookieManager", "@mozilla.org/cookiemanager;1", "nsICookieManager2");
 
 Cu.import("chrome://marionette/content/accessibility.js");
+Cu.import("chrome://marionette/content/assert.js");
 Cu.import("chrome://marionette/content/atom.js");
 Cu.import("chrome://marionette/content/browser.js");
 Cu.import("chrome://marionette/content/element.js");
 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");
@@ -643,61 +644,61 @@ GeckoDriver.prototype.setSessionCapabili
   caps = copy(newCaps, caps);
   logger.config("Changing capabilities: " + JSON.stringify(caps));
   this.sessionCapabilities = caps;
 };
 
 GeckoDriver.prototype.setUpProxy = function(proxy) {
   logger.config("User-provided proxy settings: " + JSON.stringify(proxy));
 
-  if (typeof proxy == "object" && proxy.hasOwnProperty("proxyType")) {
-    switch (proxy.proxyType.toUpperCase()) {
-      case "MANUAL":
-        Preferences.set("network.proxy.type", 1);
-        if (proxy.httpProxy && proxy.httpProxyPort){
-          Preferences.set("network.proxy.http", proxy.httpProxy);
-          Preferences.set("network.proxy.http_port", proxy.httpProxyPort);
-        }
-        if (proxy.sslProxy && proxy.sslProxyPort){
-          Preferences.set("network.proxy.ssl", proxy.sslProxy);
-          Preferences.set("network.proxy.ssl_port", proxy.sslProxyPort);
-        }
-        if (proxy.ftpProxy && proxy.ftpProxyPort) {
-          Preferences.set("network.proxy.ftp", proxy.ftpProxy);
-          Preferences.set("network.proxy.ftp_port", proxy.ftpProxyPort);
-        }
-        if (proxy.socksProxy) {
-          Preferences.set("network.proxy.socks", proxy.socksProxy);
-          Preferences.set("network.proxy.socks_port", proxy.socksProxyPort);
-          if (proxy.socksVersion) {
-            Preferences.set("network.proxy.socks_version", proxy.socksVersion);
-          }
+  assert.object(proxy);
+  if (!proxy.hasOwnProperty("proxyType")) {
+    throw new InvalidArgumentError();
+  }
+  switch (proxy.proxyType.toUpperCase()) {
+    case "MANUAL":
+      Preferences.set("network.proxy.type", 1);
+      if (proxy.httpProxy && proxy.httpProxyPort){
+        Preferences.set("network.proxy.http", proxy.httpProxy);
+        Preferences.set("network.proxy.http_port", proxy.httpProxyPort);
+      }
+      if (proxy.sslProxy && proxy.sslProxyPort){
+        Preferences.set("network.proxy.ssl", proxy.sslProxy);
+        Preferences.set("network.proxy.ssl_port", proxy.sslProxyPort);
+      }
+      if (proxy.ftpProxy && proxy.ftpProxyPort) {
+        Preferences.set("network.proxy.ftp", proxy.ftpProxy);
+        Preferences.set("network.proxy.ftp_port", proxy.ftpProxyPort);
+      }
+      if (proxy.socksProxy) {
+        Preferences.set("network.proxy.socks", proxy.socksProxy);
+        Preferences.set("network.proxy.socks_port", proxy.socksProxyPort);
+        if (proxy.socksVersion) {
+          Preferences.set("network.proxy.socks_version", proxy.socksVersion);
         }
-        break;
+      }
+      break;
 
-      case "PAC":
-        Preferences.set("network.proxy.type", 2);
-        Preferences.set("network.proxy.autoconfig_url", proxy.proxyAutoconfigUrl);
-        break;
-
-      case "AUTODETECT":
-        Preferences.set("network.proxy.type", 4);
-        break;
+    case "PAC":
+      Preferences.set("network.proxy.type", 2);
+      Preferences.set("network.proxy.autoconfig_url", proxy.proxyAutoconfigUrl);
+      break;
 
-      case "SYSTEM":
-        Preferences.set("network.proxy.type", 5);
-        break;
+    case "AUTODETECT":
+      Preferences.set("network.proxy.type", 4);
+      break;
 
-      case "NOPROXY":
-      default:
-        Preferences.set("network.proxy.type", 0);
-        break;
-    }
-  } else {
-    throw new InvalidArgumentError("Value of 'proxy' should be an object");
+    case "SYSTEM":
+      Preferences.set("network.proxy.type", 5);
+      break;
+
+    case "NOPROXY":
+    default:
+      Preferences.set("network.proxy.type", 0);
+      break;
   }
 };
 
 /**
  * Log message.  Accepts user defined log-level.
  *
  * @param {string} value
  *     Log message.
@@ -1260,20 +1261,18 @@ GeckoDriver.prototype.getWindowPosition 
  *     Object with |x| and |y| coordinates.
  */
 GeckoDriver.prototype.setWindowPosition = function(cmd, resp) {
   if (this.appName != "Firefox") {
     throw new UnsupportedOperationError("Unable to set the window position on mobile");
   }
 
   let {x, y} = cmd.parameters;
-  if (!Number.isInteger(x) || !Number.isInteger(y) ||
-      x < 0 || y < 0) {
-    throw new InvalidArgumentError();
-  }
+  assert.positiveInteger(x);
+  assert.positiveInteger(y);
 
   let win = this.getCurrentWindow();
   win.moveTo(x, y);
 
   return this.curBrowser.position;
 };
 
 /**
@@ -1565,19 +1564,17 @@ GeckoDriver.prototype.timeouts = functio
       "ms" in cmd.parameters) {
     logger.warn("Using deprecated data structure for setting timeouts");
     timeouts = {[cmd.parameters.type]: parseInt(cmd.parameters.ms)};
   } else {
     timeouts = cmd.parameters;
   }
 
   for (let [typ, ms] of Object.entries(timeouts)) {
-    if (!Number.isInteger(ms)) {
-      throw new InvalidArgumentError();
-    }
+    assert.positiveInteger(ms);
 
     switch (typ) {
       case "implicit":
         this.searchTimeout = ms;
         break;
 
       case "script":
         this.scriptTimeout = ms;
@@ -2006,20 +2003,17 @@ GeckoDriver.prototype.getElementRect = f
  *
  * @param {string} id
  *     Reference ID to the element that will be checked.
  * @param {string} value
  *     Value to send to the element.
  */
 GeckoDriver.prototype.sendKeysToElement = function*(cmd, resp) {
   let {id, value} = cmd.parameters;
-
-  if (!value) {
-    throw new InvalidArgumentError(`Expected character sequence: ${value}`);
-  }
+  assert.defined(value, `Expected character sequence: ${value}`);
 
   switch (this.context) {
     case Context.CHROME:
       let win = this.getCurrentWindow();
       let el = this.curBrowser.seenEls.get(id, {frame: win});
       yield interaction.sendKeysToElement(
           el, value, true, this.sessionCapabilities.raisesAccessibilityExceptions);
       break;
@@ -2431,29 +2425,28 @@ GeckoDriver.prototype.getScreenOrientati
  * orientation values.  If the orientation is unknown, an error will
  * be raised.
  *
  * Valid orientations are "portrait" and "landscape", which fall
  * back to "portrait-primary" and "landscape-primary" respectively,
  * and "portrait-secondary" as well as "landscape-secondary".
  */
 GeckoDriver.prototype.setScreenOrientation = function(cmd, resp) {
-  if (this.appName == "Firefox") {
-    throw new UnsupportedOperationError();
-  }
+  assert.fennec();
 
   const ors = [
     "portrait", "landscape",
     "portrait-primary", "landscape-primary",
-    "portrait-secondary", "landscape-secondary"
+    "portrait-secondary", "landscape-secondary",
   ];
 
   let or = String(cmd.parameters.orientation);
+  assert.string(or);
   let mozOr = or.toLowerCase();
-  if (ors.indexOf(mozOr) < 0) {
+  if (!ors.include(mozOr)) {
     throw new InvalidArgumentError(`Unknown screen orientation: ${or}`);
   }
 
   let win = this.getCurrentWindow();
   if (!win.screen.mozLockOrientation(mozOr)) {
     throw new WebDriverError(`Unable to set screen orientation: ${or}`);
   }
 };
@@ -2581,20 +2574,17 @@ GeckoDriver.prototype._checkIfAlertIsPre
  *
  * This method is used for custom in application shutdowns via marionette.quit()
  * or marionette.restart(), like File -> Quit.
  *
  * @param {boolean} state
  *     True if the server should accept new socket connections.
  */
 GeckoDriver.prototype.acceptConnections = function(cmd, resp) {
-  if (typeof cmd.parameters.value != "boolean") {
-    throw InvalidArgumentError("Value has to be of type 'boolean'");
-  }
-
+  assert.boolean(cmd.parameters.value);
   this._server.acceptConnections = cmd.parameters.value;
 }
 
 /**
  * Quits Firefox with the provided flags and tears down the current
  * session.
  */
 GeckoDriver.prototype.quitApplication = function(cmd, resp) {