Within Ghost.py you would find the code snippet:
if plugin_path:
for p in plugin_path:
Ghost._app.addLibraryPath(p)
From the look of things, it's expecting a path (addLibraryPath) and would do its discoveries by itself. So, give it a path containing the .xpi
Note: this isn't tested.
On my Windows 10, I typed this into a Windows address bar:
%APPDATA%\Mozilla\Firefox\Profiles
I then saw a folder named "kswjuot9.default" (it might be named another things on your PC) and clicked it. Finally, I found a folder named "extensions".
Try to give Ghost.py the full address of this "extensions" folder. I would also recommend that you use forward slashes e.g.
C:/Users/iChux/AppData/Roaming/Mozilla/Firefox/Profiles/kswjuot9.default/extensions
I saw an online link on dealing with how to extract the .xpi file