Comment 12 for bug 175904

 1. new-window : makes sense to open on current desktop
 2. new-tab: moving all tabs to current desktop is annoying; but opening new tab on other desktop is probably too confusing.

Thus: if new-tab is selected as open mode, use new-window if there is no firefox window on current desktop. optionally, we could add a pref to disable this auto-new-window feature for power users that want the ffox 2 behaviour.

(fwiw gedit behaves in a similar fashion)