That is a MP4 file played by the HTML5 media player, so plugins like Flash aren't being used in this case and disabling CTP won't help.

You can look at this pref instead:

• media.autoplay.enabled

Note that disabling autoplay can cause issues with scripts on §me websites and you may not be able to play media file or have to click play a few times.

The media.autoplay.enabled preference can cause issues on some sites and people may forget they have it set.

A alternate preference to try instead is to try toggling the media.block-play-until-visible Preference to true as it stops media from auto-starting in background tabs.

Another Preference is media.block-autoplay-until-in-foreground