Added experiment for firefox download promo

app/experiments.js

@@ -1,6 +1,28 @@
 import hash from 'string-hash';
 
-const experiments = {};
+const experiments = {
+  'SyI-hI7gT9agiH-f3f0BYg': {
+    id: 'SyI-hI7gT9agiH-f3f0BYg',
+    run: function(variant, state, emitter) {
+      state.promo = variant === 1 ? 'body' : 'header';
+      emitter.emit('render');
+    },
+    eligible: function() {
+      return (
+        !/firefox/i.test(navigator.userAgent) &&
+        document.querySelector('html').lang === 'en-US'
+      );
+    },
+    variant: function(state) {
+      return this.luckyNumber(state) > 0.5 ? 1 : 0;
+    },
+    luckyNumber: function(state) {
+      return luckyNumber(
+        `${this.id}:${state.storage.get('testpilot_ga__cid')}`
+      );
+    }
+  }
+};
 
 //Returns a number between 0 and 1
 // eslint-disable-next-line no-unused-vars
@@ -32,12 +54,12 @@ export default function initialize(state, emitter) {
       checkExperiments(state, emitter);
     });
   } else {
-    const enrolled = state.storage.enrolled;
-    enrolled.forEach(([id, variant]) => {
+    const enrolled = state.storage.enrolled.filter(([id, variant]) => {
       const xp = experiments[id];
       if (xp) {
         xp.run(variant, state, emitter);
       }
+      return !!xp;
     });
     // single experiment per session for now
     if (enrolled.length === 0) {