Funny this happened to me too. I had to drag the window and hold it at the edge of the screen until it poped there. Is there a chance that you have a modified preference for new windows in your about:config?

I cannot find a bug on this, but have only str for Mac OS. Does this happen all the time for you? It only happens periodically for me.

If so, please feel free to file a bug in http://bugzilla.mozilla.org

Not sure where about:config is? I’m on an Mac OS as well. It does happen all the time, yes (i just upgraded to the most recent Firefox version). I just tried your solution of holding as well, but that doesn't seem to work either.

I am pretty sure you do not need http://www.actualtools.com/articles/detail.php?ID=1297#2.1 but check out this add on for now: Monitor Master it lets you configure the graphics, tabs and positions of multiple windows on multiple monitors.

I keep looking for the setting in about:config.