personal experience tells me that they never work properly. They always seem to draw the size of the screen which doesn't take into account the windows bar at the bottom. I can't move the darn thing. I usually nuke it. I personally hate them and deactivated it in moz.
It will not maximize accross both monitors in a dual monitor setup, but some popup and maximize scripts default to the primary monitor, so if the current window is in the secondary monitor and your maximize script is activated, it may cause the window to shift to the primary monitor, or to open a new window in the primary monitor. Either way I find them a pain in the butt.