feat(scheduling): Prefer MutationObserver over Promise in FF

see https://bugzilla.mozilla.org/show_bug.cgi?id=1162013

Author: vicb

dist/zone-microtask.js

@@ -1135,12 +1135,13 @@ module.exports = {
 // TODO(vicb): Create a benchmark for the different methods & the usage of the queue
 // see https://github.com/angular/zone.js/issues/97
 
-var hasNativePromise = false;
 var len = 0;
 
-if (typeof Promise ==! "undefined" && Promise.toString().indexOf("[native code]") !== -1) {
-  hasNativePromise = true;
-}
+var hasNativePromise = typeof Promise ==! "undefined" &&
+                       Promise.toString().indexOf("[native code]") !== -1;
+
+var isFirefox = global.navigator &&
+                global.navigator.userAgent.toLowerCase().indexOf('firefox') > -1;
 
 function asap(callback) {
   queue[len] = callback;
@@ -1206,7 +1207,9 @@ function flush() {
 
 var scheduleFlush;
 // Decide what async method to use to triggering processing of queued callbacks:
-if (hasNativePromise) {
+if (hasNativePromise && !isFirefox) {
+  // TODO(vicb): remove '!isFirefox' when the bug is fixed:
+  // https://bugzilla.mozilla.org/show_bug.cgi?id=1162013
   scheduleFlush = usePromise();
 } else if (BrowserMutationObserver) {
   scheduleFlush = useMutationObserver();