Middle-click does nothing but makes window controls appear

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http://askubuntu.com – Just did a fresh install of Ubuntu 12.04 on my laptop and noticed that the middle-click (actually three finger-tap on the touchpad) on Firefox doesn't work as it used to. When doing it on a link it doesn't get opened on a new tab, in fact, it doesn't get opened at all. Only the (useless) window size and position controls appear, as demonstrated on terminal in the following screen shot. How do I get my tab-opening middle-clicks back? (HowTos)