If I have the focused Firefox window on my secondary monitor, I can’t open a new window. I also can’t do anything that would normally result in a new window, such as “View Source” or see “HTTP Live Headers” or open Firebug in a new window.
I have to drag the window over to my primary monitor, get the new window launched, then drag the other window back over to the second monitor.
Firefox, I love ya, but you gotta do windows better!