keyboard shortcut for switching to existing instance

[Pablo Angulo <pangard cancamusa net>]

  Hello, everybody!
  I understand one of the key points of the shell is to encourage the
user to use existing instances of applications instead of opening new
ones. I use a lot global shortcuts to open new instances of
applications. I wonder if I can switch to an existing application of a
specific kind. Note that alt+tab doesn't do this, because it switches
through all open apps.
  So is there a way to switch to an existing instance of an application
with a keyboard shortcut?. For example, Super+f opens a new firefox
window, but Super+Shift+f cycles through existing firefox windows (or
viceversa), Super+e and Super+shift+e do the same with the text editor,
etcetera.
  Specifically, I recently discovered the quake-style terminals yakuake
and guake, and I'd love to do the same with the browser and the text
editor. If I had shortcuts as described above, I wouldn't need a special
terminal, I could use gnome-terminal just the way I use guake. Some apps
work this way by default, like thunderbird. Wherever I run the command
thunderbird, the only thunderbird instance moves the current workspace
and gain focus. For apps that need several instances, more care is needed.
  Regards, and congratulations for your great work.