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.