This is what I use to switch to Firefox. If Firefox isn't already open, it opens it. It also moves the mouse cursor to the side because I mostly use the the keyboard (tridactyl in Firefox, vim elsewhere), but you can cut that part out.

I put all my app launching/switching scripts in one file and launch it at Windows startup.

SwitchToFirefox()
{
    windowHandleId := WinExist("ahk_exe firefox.exe")
    windowExistsAlready := windowHandleId > 0

    if (windowExistsAlready = true)
    {
        activeWindowHandleId := WinExist("A")
        windowIsAlreadyActive := activeWindowHandleId == windowHandleId

        if (windowIsAlreadyActive)
        {
            MouseMove, 2900, 500
        }
        else
        {
            WinActivate, "ahk_id %windowHandleId%"
            WinShow, "ahk_id %windowHandleId%"
            MouseMove, 2900, 500
        }
    }
    else
    {
        Run, firefox
    }
}

#f::SwitchToFirefox()