I just upgrade to F14 from F10 on a dual montor setup.
Firefox behaives differently on F14 than on F10.
On F10 it followed the mouse pointer's location and show up on the correct monitor
When maximized at close, it is always displayed on the left monitor.
When not maximized at close it will open on the monitor where the mouse pointer is located.
There are some guidlines about minimize/maximize Firefoxe before closing and it seems to
work in some case, but not for me.