Firefox supports loading plugins like flash on demand on the Addons settings page. Additionally, Firefox has a setting "media.autoplay.enabled" to prevent HTML5

Firefox supports loading plugins like flash on demand on the Addons settings page. Additionally, Firefox has a setting "media.autoplay.enabled" to prevent HTML5 media from playing automatically. However, some websites assume autoplay succeeded and behave wrongly. For example, YouTube's paused/play button state is backwards.