Applescript automate double keystroke

It looks like the Finder needs the absolute path to the home folder instead of the relative path. Instead of starting the path with ~/, it needs to start with /Users/username/.

Instead of hardcoding the username into the script, you can have AppleScript build the absolute path on the fly:

set homePath to POSIX path of (path to home folder)

Then you can replace "~/ with homePath & "

For example:

if exists "~/Downloads/Conversion/" & cbUsername & ".flv" as POSIX file then

would become

if exists homePath & "Downloads/Conversion/" & cbUsername & ".flv" as POSIX file then

Alternatively, if you only use ~ with the path ~/Downloads/Conversion/, you could instead change that whole path to a variable:

set cbPath to POSIX path of (path to home folder) & "Downloads/Conversion/"

Then the final script would be:

set cbPath to POSIX path of (path to home folder) & "Downloads/Conversion/"

tell application "Finder"
    if exists cbPath & cbUsername & ".flv" as POSIX file then
        set x to 1
        repeat
            set newCbFilename to cbUsername & "_" & x as string
            if exists cbPath & newCbFilename & ".flv" as POSIX file then
                set x to x + 1
            else
                exit repeat
            end if
        end repeat
        copy newCbFilename to finalCbFilename
        display dialog "Filename already exists " & "File will be named: " & finalCbFilename & ".flv" buttons "OK" default button "OK" with title "Error" with icon caution
    else
        copy cbUsername to finalCbFilename
    end if
end tell

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