* Use wildcard origin when sending messages to or from app.html
   served from a background page.

 * In embed.js, access the 'chrome' global via 'chrome' rather than
   window.chrome. In Firefox the 'chrome' global is not attached to
   the `window` object.

 * Work-around difference in how content script results are reported
   in Firefox (an object) vs Chrome (an array of objects, one per
   frame).

 * Avoid trying to call APIs which do not exist in Firefox.