Aero Peek / Superbar doesn't work / Firefox (3.6) tab-preview doesn’t work in Windows 7

After upgrading to the latest Firefox (FF) 3.6, the Aero Peek / Superbar doesn’t show except one instance of FF and tab-preview is not working with FF.

After upgrading my browser, the setting for browser.taskbar.previews.enable, in About:Config, is changed from true to false...

Just open FF and type in about:config in the address bar, click I'll be careful, I promise!, button and search for browser.taskbar.previews.enable. If the value is set to false, double-click it to set it to true, restart your browser.