Starting with Firefox 20, the ‘Esc’ key will no longer stop a page from loading or stop animated GIFs from animating. This change goes back to November of 2010 with Bug 614304 (ESC key aborts XMLHttpRequest and WebSocket). Without getting into too much technical details, pressing the Esc key can cause major problems for sites that use Web Apps that are coded in Ajax or use jQuerry. With the growing popularity and number of web apps came a great number of users accidentally hitting the escape key. So effective with Firefox 20 the Esc key will no longer stop anything, it simply won’t do anything.
There was a proposal (bug 824248) to remap the old Esc key behavior to SHIFT+Esc. However, it seems that Mozilla does not want to make this change as a result the bug has been marked as RESOVLED WONTFIX. There is an add-on that will allow users to press SHIFT+Esc to stop animated gifs (and anything else on the page). The add-on is SuperStop and only works with Firefox 20 (moving into the Aurora branch next week) or newer.