how to change the display size of the firefox-os simulator? -

i'm running firefox os 2.0 simulator via firefox webide.

the mdn firefox os doc states 320x480 pixels device size, of supported devices have higher resolutions available.

i able change device screen size can test application in simulator different display sizes - how done?

here image webide showing device settings disabled:

i'm using firefox version 39.

simulator configuration made available in firefox 42. gives access options window size:

switching nightly access firefox 42 best path now, since number of related bugs fixed in version.