The biggest problem is that the SCREENCOUNT() function returns the number of logical displays, not physical -- so the value will be 2 in both your cases.
But a quick test suggests that when you are mirroring the external display on your internal the result of SCREEN(Internal,Width) is 0 -- if that's the same for you then a solution might be:
Firefox for 2 or 3 Screens.kmmacros (5.8 KB)
...moving the frontmost Firefox window to full size on the appropriate display.
This is a complete guess, since I don't have multiple displays (and an Air can only drive one external [without a lot of messing with DisplayLink, before anyone mentions that]) so I'd love to know how you get on!