All platforms with Firefox: The SiteLauncher extension adds a keyboard launcher for your favourite bookmarks in Firefox. Hit the extension's default launcher shortcut—Ctrl+Alt on my Mac, Ctrl+Space on Windows—then hit the key you see next to your bookmark of choice in the dark prompt panel shown below.
Once you've memorized the keys that go along with your most-used bookmarks, you can just hit the "Direct to Site Hotkey" plus the letter and off you go, no prompt involved. Of course all the URLs and keys associated with them are configurable. Quicksilver lovers will recognise this interface, which is a little easier to get used to than straight-up Firefox bookmark keywords. SiteLauncher is a free download that will work wherever Firefox does; thanks to the creators for including Lifehacker as a default entry.