When searching for a theme on addons.mozilla.org, there is no way to see if a theme is animated or what the animation looks like. There is already a preview of the theme visible, so my suggestion is that the preview shows the animation aswell. It would also be helpful if it was possible to search for animated themes through a category or tag. Currently it seems like you have to hope that the theme creator has put "animated" in the theme-name.
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This is the right place for posting, @HumanTownGrove 🙌
There's also a similar (ish) idea here: a preview for theme