(edit3)
Can you tell me which POST engines aren't working? I'll take a look.

1.2.1b6 in the beta section fixed a bug where {searchTerms} in POST was only replaced in the form params and not if it was also included in the url params. If that was causing your issues, give the beta a try.

I've updated the readme at github to better reflect your experience setting up the python script. Let me know if there's anything else that would be helpful for other users installing the native app

(edit)
The instructions to put the .json in /usr/lib or /usr/lib64 were taken directly from the Firefox documentation for the native app API. On the chance you see this edit, would you mind opening a ticket at github and elaborating on why this incorrect?

(2) Hmm, yeah that probably should be in Unix (LF) Oops, I'll update
(3) That's what the Firefox Native Messaging API requires. Nothing I can do there.

Glad to hear you got it working. You might be the first that used the python script. Guess I'll have to keep it updated :(