Don't lose your Youtube fullscreen when you try to be productive in your other (dual) monitor in Firefox
By Dan Herbert: a bookmarklet that will take you from the standard Youtube video viewing page to a fullscreen popout so you can work in another browser window on the other monitor without losing your fullscreen video like you would if you just hit the regular Flash fullscreen button. One caveat: you have to be on the actual Youtube video page, not viewing an embedded video or on a user’s channel page.
Here is the actual bookmarklet you can drag to your address bar.
NOTE: Chrome doesn’t kill your fullscreen when you switch out.