The code that you are looking for is somewhat similar to the example they give in the docs for WinActivate
So modifying that example for your purpose, and adding conditionals would give you:
CapsLock & w::
if WinExist("ahk_exe firefox.exe")
if WinActive("ahk_exe firefox.exe")
WinMinimize
else
WinActivate
else
Run firefox.exe
Return
Take note that this script will currently only minimize a Firefox window if it is the currently active window. If you need it to minimize a Firefox in the background, the script would potentially be a bit more complex since you could possibly have multiple Firefox windows open, and you would need to provide conditions and logic to handle cases like those. However, if you need this functionality, describe what behavior you would like to occur if this condition occurs, and I can work on it.