Windows/Mac/Linux: ShortcutKey2URL is an ideal Firefox add-on for anyone who visits certain sites so often, they'd save serious time with just two key motions instead of typing out a URL (or learning some AwesomeBar magic).
ShortcutKey2URL offers just a single option dialog, where you give a frequently-visited site a single letter shortcut, name it, and then provide the URL. You can pre-set this shortcut to open in your existing tab, in a new one, or search to see if it's already open in another tab, then open as a refresh in that one. You activate all of your one-letter shortcuts by first hitting your activation combo, which is Ctrl+Q by default, but can easily be changed. You can also register any site you visit as a ShortcutKey by hitting the grey arrow in your address bar—but, in a bad design move, that arrow never goes away. If peripheral clutter doesn't drive you nuts, though, this could be a really helpful keyboard enabler. ShortcutKey2URL is a free download, works wherever Firefox does.