I'm not sure I understand the question... bringing the desktop to focus requires "one extra action" regardless (either clicking the taskbar or empty space on the desktop itself, or alt tabbing to it), so you're already out of luck.
Macintosh has spotlight, but that requires two keystrokes (propeller+spacebar), Windows cuts it down to just one key (windows key, or CTRL+Esc on a keyboard without a WinKey). If you want to be 'in' firefox, and just start whacking away, there's very few options available, as almost any program will be trying to interpret the keystrokes for its own use (photoshop shortcut keys, jumping to text on a webpage, whatever).
Pinning buttons to the startmenu or taskbar is an option, and then using WinKey+ to launch them (from within any app), but that's as close as you'll get I think.