diff --git a/q.js b/q.js
index fa300e42..0317c6ef 100644
--- a/q.js
+++ b/q.js
@@ -262,49 +262,10 @@ var object_keys = Object.keys || function (object) {
     return keys;
 };
 
-var object_toString = uncurryThis(Object.prototype.toString);
-
 function isObject(value) {
     return value === Object(value);
 }
 
-// generator related shims
-
-// FIXME: Remove this function once ES6 generators are in SpiderMonkey.
-function isStopIteration(exception) {
-    return (
-        object_toString(exception) === "[object StopIteration]" ||
-        exception instanceof QReturnValue
-    );
-}
-
-// FIXME: Remove this helper and Q.return once ES6 generators are in
-// SpiderMonkey.
-var QReturnValue;
-if (typeof ReturnValue !== "undefined") {
-    QReturnValue = ReturnValue;
-} else {
-    QReturnValue = function (value) {
-        this.value = value;
-    };
-}
-
-// Until V8 3.19 / Chromium 29 is released, SpiderMonkey is the only
-// engine that has a deployed base of browsers that support generators.
-// However, SM's generators use the Python-inspired semantics of
-// outdated ES6 drafts.  We would like to support ES6, but we'd also
-// like to make it possible to use generators in deployed browsers, so
-// we also support Python-style generators.  At some point we can remove
-// this block.
-var hasES6Generators;
-try {
-    /* jshint evil: true, nonew: false */
-    new Function("(function* (){ yield 1; })");
-    hasES6Generators = true;
-} catch (e) {
-    hasES6Generators = false;
-}
-
 // long stack traces
 
 var STACK_JUMP_SEPARATOR = "From previous event:";
@@ -1184,29 +1145,15 @@ function async(makeGenerator) {
         // when verb is "throw", arg is an exception
         function continuer(verb, arg) {
             var result;
-            if (hasES6Generators) {
-                try {
-                    result = generator[verb](arg);
-                } catch (exception) {
-                    return reject(exception);
-                }
-                if (result.done) {
-                    return result.value;
-                } else {
-                    return when(result.value, callback, errback);
-                }
+            try {
+                result = generator[verb](arg);
+            } catch (exception) {
+                return reject(exception);
+            }
+            if (result.done) {
+                return result.value;
             } else {
-                // FIXME: Remove this case when SM does ES6 generators.
-                try {
-                    result = generator[verb](arg);
-                } catch (exception) {
-                    if (isStopIteration(exception)) {
-                        return exception.value;
-                    } else {
-                        return reject(exception);
-                    }
-                }
-                return when(result, callback, errback);
+                return when(result.value, callback, errback);
             }
         }
         var generator = makeGenerator.apply(this, arguments);
@@ -1228,36 +1175,6 @@ function spawn(makeGenerator) {
     Q.done(Q.async(makeGenerator)());
 }
 
-// FIXME: Remove this interface once ES6 generators are in SpiderMonkey.
-/**
- * Throws a ReturnValue exception to stop an asynchronous generator.
- *
- * This interface is a stop-gap measure to support generator return
- * values in older Firefox/SpiderMonkey.  In browsers that support ES6
- * generators like Chromium 29, just use "return" in your generator
- * functions.
- *
- * @param value the return value for the surrounding generator
- * @throws ReturnValue exception with the value.
- * @example
- * // ES6 style
- * Q.async(function* () {
- *      var foo = yield getFooPromise();
- *      var bar = yield getBarPromise();
- *      return foo + bar;
- * })
- * // Older SpiderMonkey style
- * Q.async(function () {
- *      var foo = yield getFooPromise();
- *      var bar = yield getBarPromise();
- *      Q.return(foo + bar);
- * })
- */
-Q["return"] = _return;
-function _return(value) {
-    throw new QReturnValue(value);
-}
-
 /**
  * The promised function decorator ensures that any promise arguments
  * are settled and passed as values (`this` is also settled and passed