Escape in Firefox not stopping animations

[Lars Noodén]

Earlier in Firefox, it was possible to stop animations by pressing
Escape.  Now it won't stop anything.  Are other people seeing this
problem?

Here is one animated GIF that won't stop:

http://laughingsquid.com/internet-census-2012-an-illegally-obtained-animated-map-of-the-web/

But there are others including more and more ads.  I know that escape
used to stop at least the animated GIFs.  However, the current version
of Firefox seems broken and have no way to stop animated GIFs from
cycling.

How can I get FF to stop running the animation?

Regards,
/Lars