There's a larger and larger number of websites that auto-play completely unrelated videos when opening a page on their website. In my opinion, a very user unfriendly business.
Thankfully Firefox has an extension for this, FlashStopper: https://addons.mozilla.org/en-US/firefox/addon/flashstopper/
Before and after installing, open for example this page on the Reuters website: http://www.reuters.com/article/us-twitter-lawsuit-idUSKBN1782PH