I was having this issue and it came down to this.
I have multiple monitors. On my right hand one, I have my nagios/ganglia window fit to the screen.
I use other FF windows on other monitors. If the 'fit to screen' window is active when I open a new window on another screen, it opens 'fit to screen'
So, I have to try not to have had that window active before creating a new window. PITA