If the status bar is hidden, the tree panel will hang off of the main window's title bar, probably obscuring useful controls. Need to check if the status bar is hidden and if it is, attach the panel to the bottom of the browser element. This might be a better location to hang this panel anyway.
this will be fixed by the "better placement bug" https://bugzilla.mozilla.org/show_bug.cgi?id=583037
i believe everything about this bug is out of date now.