Superstop - get rid of animations in gifs in Firefox
SuperStop is very simple: it adds a Shift+Esc shortcut that stops active WebSocket connections, XMLHttpRequests, and any animated images, regardless of the state of the Firefox Stop button. This matches the behavior of the Esc button in versions of Firefox prior to Firefox 20 (see bug 824248).
Read more: Link - SuperStop :: Add-ons for Firefox