--- a/testing/specialpowers/content/SpecialPowersObserverAPI.js
+++ b/testing/specialpowers/content/SpecialPowersObserverAPI.js
@@ -1,35 +1,34 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
 "use strict";
 
 Components.utils.import("resource://gre/modules/Services.jsm");
 
 if (typeof(Ci) == 'undefined') {
   var Ci = Components.interfaces;
 }
 
 if (typeof(Cc) == 'undefined') {
   var Cc = Components.classes;
 }
 
-/**
- * Special Powers Exception - used to throw exceptions nicely
- **/
-this.SpecialPowersException = function SpecialPowersException(aMsg) {
+this.SpecialPowersError = function(aMsg) {
+  Error.call(this);
+  let {stack} = new Error();
   this.message = aMsg;
-  this.name = "SpecialPowersException";
+  this.name = "SpecialPowersError";
 }
+SpecialPowersError.prototype = Object.create(Error.prototype);
 
-SpecialPowersException.prototype = {
-  toString: function SPE_toString() {
-    return this.name + ': "' + this.message + '"';
-  }
+SpecialPowersError.prototype.toString = function() {
+  return `${this.name}: ${this.message}`;
 };
 
 this.SpecialPowersObserverAPI = function SpecialPowersObserverAPI() {
   this._crashDumpDir = null;
   this._processCrashObserversRegistered = false;
   this._chromeScriptListeners = [];
 }
 
@@ -216,17 +215,17 @@ SpecialPowersObserverAPI.prototype = {
     } catch(e) {
       /* The channel is not a nsIHttpCHannel, but that's fine */
       dump("-*- _readUrlAsString: Got an error while fetching " +
            "chrome script '" + aUrl + "': (" + e.name + ") " + e.message + ". " +
            "Ignoring.\n");
     }
 
     if (status == 404) {
-      throw new SpecialPowersException(
+      throw new SpecialPowersError(
         "Error while executing chrome script '" + aUrl + "':\n" +
         "The script doesn't exists. Ensure you have registered it in " +
         "'support-files' in your mochitest.ini.");
     }
 
     return output;
   },
 
@@ -242,29 +241,29 @@ SpecialPowersObserverAPI.prototype = {
       case "SPPrefService": {
         let prefs = Services.prefs;
         let prefType = aMessage.json.prefType.toUpperCase();
         let prefName = aMessage.json.prefName;
         let prefValue = "prefValue" in aMessage.json ? aMessage.json.prefValue : null;
 
         if (aMessage.json.op == "get") {
           if (!prefName || !prefType)
-            throw new SpecialPowersException("Invalid parameters for get in SPPrefService");
+            throw new SpecialPowersError("Invalid parameters for get in SPPrefService");
 
           // return null if the pref doesn't exist
           if (prefs.getPrefType(prefName) == prefs.PREF_INVALID)
             return null;
         } else if (aMessage.json.op == "set") {
           if (!prefName || !prefType  || prefValue === null)
-            throw new SpecialPowersException("Invalid parameters for set in SPPrefService");
+            throw new SpecialPowersError("Invalid parameters for set in SPPrefService");
         } else if (aMessage.json.op == "clear") {
           if (!prefName)
-            throw new SpecialPowersException("Invalid parameters for clear in SPPrefService");
+            throw new SpecialPowersError("Invalid parameters for clear in SPPrefService");
         } else {
-          throw new SpecialPowersException("Invalid operation for SPPrefService");
+          throw new SpecialPowersError("Invalid operation for SPPrefService");
         }
 
         // Now we make the call
         switch(prefType) {
           case "BOOL":
             if (aMessage.json.op == "get")
               return(prefs.getBoolPref(prefName));
             else 
@@ -301,17 +300,17 @@ SpecialPowersObserverAPI.prototype = {
           case "unregister-observer":
             this._removeProcessCrashObservers();
             break;
           case "delete-crash-dump-files":
             return this._deleteCrashDumpFiles(aMessage.json.filenames);
           case "find-crash-dump-files":
             return this._findCrashDumpFiles(aMessage.json.crashDumpFilesToIgnore);
           default:
-            throw new SpecialPowersException("Invalid operation for SPProcessCrashService");
+            throw new SpecialPowersError("Invalid operation for SPProcessCrashService");
         }
         return undefined;	// See comment at the beginning of this function.
       }
 
       case "SPPermissionManager": {
         let msg = aMessage.json;
 
         let secMan = Services.scriptSecurityManager;
@@ -333,18 +332,18 @@ SpecialPowersObserverAPI.prototype = {
           case "test":
             let testPerm = Services.perms.testPermissionFromPrincipal(principal, msg.type, msg.value);
             if (testPerm == msg.value)  {
               return true;
             }
             return false;
             break;
           default:
-            throw new SpecialPowersException("Invalid operation for " +
-                                             "SPPermissionManager");
+            throw new SpecialPowersError(
+              "Invalid operation for SPPermissionManager");
         }
         return undefined;	// See comment at the beginning of this function.
       }
 
       case "SPSetTestPluginEnabledState": {
         var plugin = getTestPlugin(aMessage.data.pluginName);
         if (!plugin) {
           return undefined;
@@ -372,30 +371,30 @@ SpecialPowersObserverAPI.prototype = {
           case "debug-customizations":
             {
               let scope = {};
               Components.utils.import("resource://gre/modules/UserCustomizations.jsm", scope);
               scope.UserCustomizations._debug = aMessage.json.value;
               return;
             }
           default:
-            throw new SpecialPowersException("Invalid operation for SPWebAppsService");
+            throw new SpecialPowersError("Invalid operation for SPWebAppsService");
         }
         return undefined;	// See comment at the beginning of this function.
       }
 
       case "SPObserverService": {
         switch (aMessage.json.op) {
           case "notify":
             let topic = aMessage.json.observerTopic;
             let data = aMessage.json.observerData
             Services.obs.notifyObservers(null, topic, data);
             break;
           default:
-            throw new SpecialPowersException("Invalid operation for SPObserverervice");
+            throw new SpecialPowersError("Invalid operation for SPObserverervice");
         }
         return undefined;	// See comment at the beginning of this function.
       }
 
       case "SPLoadChromeScript": {
         let url = aMessage.json.url;
         let id = aMessage.json.id;
 
@@ -438,19 +437,20 @@ SpecialPowersObserverAPI.prototype = {
           },
           configurable: true
         });
 
         // Evaluate the chrome script
         try {
           Components.utils.evalInSandbox(jsScript, sb, "1.8", url, 1);
         } catch(e) {
-          throw new SpecialPowersException("Error while executing chrome " +
-                                           "script '" + url + "':\n" + e + "\n" +
-                                           e.fileName + ":" + e.lineNumber);
+          throw new SpecialPowersError(
+            "Error while executing chrome script '" + url + "':\n" +
+            e + "\n" +
+            e.fileName + ":" + e.lineNumber);
         }
         return undefined;	// See comment at the beginning of this function.
       }
 
       case "SPChromeScriptMessage": {
         let id = aMessage.json.id;
         let name = aMessage.json.name;
         let message = aMessage.json.message;
@@ -466,17 +466,17 @@ SpecialPowersObserverAPI.prototype = {
         let mm = aMessage.target
                          .QueryInterface(Ci.nsIFrameLoaderOwner)
                          .frameLoader
                          .messageManager;
         let msg = aMessage.data;
         let op = msg.op;
 
         if (op != 'clear' && op != 'getUsage') {
-          throw new SpecialPowersException('Invalid operation for SPQuotaManager');
+          throw new SpecialPowersError('Invalid operation for SPQuotaManager');
         }
 
         let uri = this._getURI(msg.uri);
 
         if (op == 'clear') {
           if (('inBrowser' in msg) && msg.inBrowser !== undefined) {
             qm.clearStoragesForURI(uri, msg.appId, msg.inBrowser);
           } else if (('appId' in msg) && msg.appId !== undefined) {
@@ -505,18 +505,17 @@ SpecialPowersObserverAPI.prototype = {
         } else {
           qm.getUsageForURI(uri, callback);
         }
 
         return undefined;	// See comment at the beginning of this function.
       }
 
       default:
-        throw new SpecialPowersException("Unrecognized Special Powers API");
+        throw new SpecialPowersError("Unrecognized Special Powers API");
     }
 
     // We throw an exception before reaching this explicit return because
     // we should never be arriving here anyway.
-    throw new SpecialPowersException("Unreached code");
+    throw new SpecialPowersError("Unreached code");
     return undefined;
   }
 };
-