MacOS – How to execute AppleScript from Textedit

Try this

tell application "Firefox"
    activate
    tell application "System Events"
        keystroke tab
        keystroke (key code 125)
        keystroke (key code 125)
        keystroke (key code 125)        
    end tell
end tell

You might not need to activate Firefox if it's the current active app. Them you just need :

tell application "System Events"
    keystroke tab
    keystroke (key code 125)
    keystroke (key code 125)
    keystroke (key code 125)        
end tell

Try sending the Activate command outside the System Event tell, that would be the standard method.

tell application "Canon MX510 series" to activate
my testAppRunning()
tell application "System Events"    
    key code 20 using command down
end tell

on testAppRunning()
    set test to 0
    repeat while test = 0
        log test
        tell application "System Events" to set test to count (every process whose name is "Canon MX510 series")
        delay 2
        --/*we do this even if active because it doesn't naturally come to front*/
        try
            tell application "Canon MX510 series" to activate
        end try
        if (test > 0) then exit repeat
    end repeat
end testAppRunning