Works perfectly. I even got it to be just Esc instead of Shift+Esc! Rated 5 out of 5 stars
This works as advertised for me, and I even learned how to change the shortcut from Shift+Esc to just Esc! Here's how:
1. Install Superstop
2. Install keyconfig: http://forums.mozillazine.org/viewtopic.php?t=72994
3. Go into keyconfig (keyboard shortcut: Ctrl+Shift+F12)
4. Look for "superstop-shortcut" and select it
5. Click in the text box where you see "Shift+Esc"
6. Press Esc (so don't type it)
7. Keyconfig will say "This shortcut is already used by Stop. It might still work, though". This is correct: it does!
9. Click OK
10. Click Apply
11. Click Close
Thanks to this, I am now using Firefox 21.0 and I am able to stop animated GIFs by pressing just Esc! Naturally, I'd love to see the ability to toggle between the new and old behaviors of the Esc key in either Tools > Options or about:config, but for now this is definitely good enough for me!
I learned this by complaining in this bug: