Improve use of scenes in /firefox/new.

See bug 919788.

This makes a few changes:

 * if you arrive on the page directly from #download-fx the animation is suppressed.
 * dowload is initiated in an iframe so current page CSS images don't get cancelled before loading.
 * scene2 is suppressed on Android
 * fixed bug in IE < 9 going directly to #download-fx
 * improve inline commenting
 * use LESS-CSS mixin for transitions in CSS.
Open popul before sending GA tracking.

Seems to prevent popup blocker in some cases.
Merge pull request #1275 from gauthierm/improve-use-of-scenes-on-new

Improve use of scenes in /firefox/new.