(I know this is really late, but HN has been really aggressive about "submitting too fast" lately, and a half hour's wait didn't convince it to let me try again.)
I'm sorry; you're right, I misunderstood your needs, and thought that you wanted GIFs not to play at all. As with "there's an app for that", the answer to most Firefox needs is "there's an extension for that". I'm not in touch with the current Firefox extension ecosystem, but some Googling suggests that https://addons.mozilla.org/en-US/firefox/addon/toggle-animat... should fill the need.