Youtube refreshes when clicking on it's tab?

Whenever I view youtube or nicovideo and click a link to a video and use open link in background tab and ignore it for a bit, then click to the new tab the video refreshes, or use open link in foreground tab click to another tab and come back to the new tab the video still refreshes. Now this also happened with the previous version of firefox, not sure when it started happening but it's become an annoyance. == URL of affected sites == http://www.youtube.com, http://www.nicovideo.jp