Sigh, I guess it's time to enable Click-to-play again[1]. I wish there were a way to just automatically pause on load, without needing to completely disable flash.
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.