Macbook's display and monitor's display don't mirror each other. So basically 2 screens
3 Display setup:
MacBook's display mirrors one monitor (main display), and the second monitor is my second screen.
WHAT I WANT:
Create a setup with KM that does the following:
For 2 Display mode:
When I connect my monitor, move Firefox to my laptop screen and keep other applications on the main screen (monitor)
For 3 Display mode:
Whenever I connect my second monitor, move the Firefox window to this new monitor and keep everything else the same. In this setup, my laptop mirrors the initial primary monitor
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:
...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!