Up until Firefox 19, you could stop an animated GIF by hitting the Escape key. This is no longer possible in Firefox 20. Furthermore, if an animated GIF is still loading and if the load is interrupted by pressing the Escape key or clicking the Stop button, the GIF is removed instead of keeping the partially loaded image. This can potentially break the page layout.
The new behavior was adopted by Firefox 20 when developers fixed an old bug related to the Esc key. Previously, pressing the Escape key not only stopped GIF animations but also terminated all XMLHttpRequests and WebSocket connections of the current tab. This caused problems with many modern web applications such AJAX that doesn’t retry requests once aborted, leading to broken behavior.
In Firefox 20, Mozilla removed the ability to terminate connections by hitting the Esc key. Escape now only has an effect when the stop button is visible. Since the stop button doesn’t stop animating GIFs, hitting the Esc key also has no effect on animations. If you are reading this is Firefox 20, you can test this behavior by trying to stop the animated GIF below.
Solutions: There are three ways to bring back the old behavior.
SuperStop is restart-less add-on that adds a Shift+Esc shortcut that stops active WebSocket connections, XMLHttpRequests, and any animated images, regardless of the state of the Firefox Stop button.
Active Stop Button makes the Stop button be never inactive so that you can stop gif animation by mouse click, as well as with the Esc key. The add-on can be further configured to stop all tabs with single click, double click, ctrl+click or right click on Stop button.