[bug 1100861] Temporarily disable firefox desktop intro anim on Firefox 35 (OSX)

Due to a Firefox graphics bug on OSX, the /firefox/desktop SVG/CSS intro animation is currently a bit broken on version 35 and up.

I filed [bug 1083079](https://bugzilla.mozilla.org/show_bug.cgi?id=1083079) a while ago and we found the cause of the regression. The bug is being worked on, but in the mean time this disables the animation for OSX users on Firefox 35 or greater. Hopefully it can be resolved soon and we can enable it again.

I also took this opportunity to simplify the animation loading, by using the regular window `load` event instead of waiting for the individual logo to load.