Issue originally raised here https://support.mozilla.org/en-US/questions/1016389 However the ability to save forwards into the original folder the same as a reply would enhance the functionality of the existing option. And make SMTP activity consistent. The existing option implies "On Send" except it is not on forward send only reply send.
I'm the one that submitted the question on the support forum that Matt responded to. I have used M$ Outlook (Office product) that includes support for both replies and forwards and found it to be most useful. Both are features I find lacking in Apple Mail. Thunderbird is halfway there. Thanks, Matt!