Want YouTube to pause your videos automatically? YouTube Smart Pause automatically pauses currently playing videos on YouTube once you leave the page and resumes playback as soon as you return. This is a simple and clean solution to a problem that I know a lot of YouTube users have.
Via the options, you can set it this is active by default (pausing the video) when you open a new tab. Also, the smart pause feature will go into effect once a tab/window gets hidden or minimized too. If needed you can delay the pausing of the video by X number of seconds. Want a visual representation? Choose to have the favicon change for the tab when you have the video paused.
Love this add-on to death, it is a good one. You can pick up the YouTube Smart Pause add-on from the Firefox Add-on website.