Rated 4 out of 5 stars
"I suppose many people want to browse website and meanwhile watch youtube video"
The best addon for that is PopVideo (https://addons.mozilla.org/en-US/firefox/addon/popvideo/)
... 10x better than splitting the browser frame, can also have alwaysOnTop enabled on the popout. Of course it just pops the video, so if you still want to read youtube comments then yeah Tile View.