Replace chrome with generalized browser API.

app/scripts/background.js

@@ -37,8 +37,8 @@ function showUnconfirmedTx (txParams, txData, onTxDoneCb) {
 
 // On first install, open a window to MetaMask website to how-it-works.
 
-chrome.runtime.onInstalled.addListener(function (object) {
-  chrome.tabs.create({url: 'https://metamask.io/#how-it-works'})
+extension.runtime.onInstalled.addListener(function (object) {
+  extension.tabs.create({url: 'https://metamask.io/#how-it-works'})
 })
 
 //