Yes, Firefox 20 broke your ESC key to stop animations
No, you're not going crazy. Firefox 20 did in fact disable your ability to hit ESC and stop those stupid looping GIF animations. If you've been wondering why it seems like every site suddenly figured out how to eat your ESC key, that's actually what's going on.
There's an add-on which will bring it back, but it only binds to shift-ESC, so it's no longer a simple "jab at the corner of the keyboard" move, but instead requires a modifier key, too.
I can't even remember how far back the "ESC to stop animating garbage" goes back in the world of web browsers. I used to purposely binary-mangle my Netscape binaries way back in the day to make them unable to pick up on the extensions which specified looping. Then they added ESC-to-disable at some point, and life was good. I could watch animated GIFs if I wanted, and I could stop the annoying ones too.
Now, this is the state of affairs. How stupid.