[mythbuntu] Run Firefox in Dual Monitor Environment (3840x1080)

How to run Firefox in full screen mode with resolution of 3840x1080?
When I press F11, it only have full screen on one monitor (1920x1080), it caot extend to dual monitors.

xrandr
=====
Screen 0: minimum 8 x 8, current 3840 x 1080, maximum 32767 x 32767
HDMI1 coected 1920x1080+1920+0
VGA1 coected 1920x1080+0+0 (Left of HDMI1)

I can move firefox across each screens. But I have no idea how to run firefox with resolution of 3840x1080 (Extend to dual monitors).

Thanks,
Wilson