Using Firefox 4 on Windows 7 with dual monitors I noticed some odd behavior when moving and maximizing windows between my two monitors.
I had moved Firefox to the left monitor and now wanted to move it back to the right hand monitor. I would reduce Firefox from being maximized and then move it to the other monitor and maximize it before closing it. When I opened Firefox again it would open on the left monitor instead of the right monitor.
The fix was simple enough – reduce Firefox from its maximized state on the left monitor, drag it to the right hand monitor, and then close it (un-maximized). This time Firefox would open on the correct monitor and could be maximized and closed without further issue.