Fix popup behavior for Firefox

Firefox does not support the `focused` parameter when opening a new window, and we don't actually require it for Chrome either, new popups are at the foreground by default already.

app/scripts/lib/extension.js

@@ -11,4 +11,7 @@
  */
 
 const Extension = require('./extension-instance')
-module.exports = new Extension()
+const instance = new Extension()
+window.METAMASK_EXTENSION = instance
+module.exports = instance
+

app/scripts/lib/notifications.js

@@ -22,10 +22,12 @@ function show () {
       extension.windows.create({
         url: 'notification.html',
         type: 'popup',
-        focused: true,
         width,
         height,
       })
+      .catch((reason) => {
+        log.error("failed to create poupup", reason)
+      })
     }
   })
 }