It is an animated gif. You used to be able to just press escape to pause them, but Firefox and Chrome have both removed that behavior. Instead you can install a plugin, or in Firefox you can set `image.animation_mode` to:

`once` - play once on load then stop.

`none` - do not play on load

`normal` - loop forever if the gif says to

With the popularity of animated "emoji" on some forums these days, I couldn't function without some way to globally disable animated gifs.