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You can try Vertical Tabs in Firefox now
(www.ghacks.net)
A community for discussion about Mozilla Firefox.
YouTube for one, and if you were to place two windows side by side you'd loose a lot of space on duplicate tab bars.
Not sure what you're actually losing on YouTube though... You go fullscreen quickly anyway. And if I put two windows side by side I just disable the tab because I'm likely doing it for a reason to have two specific pages on them, actually giving more screenspace because I don't have the bar at the top either.