If you temporarily set about:config "image.animation_mode" to the value "none", and then restart Firefox, does it stop that video from autoplaying? If so, it's

If you temporarily set about:config "image.animation_mode" to the value "none", and then restart Firefox, does it stop that video from autoplaying? If so, it's because they're using an animated GIF, which isn't a video. (You can right-click the value you modified in about:config to Reset it back to the default, so that animated GIFs work again, after you're done testing that.)