The setup for developing extensions (at least) for Firefox seems unnecessarily complicated.
Why can't you just create a manifest.json and whatever.js in a extensions or private extensions folder in your Firefox userspace?
For example:
"C:\Users\Mutzu\AppData\Local\Mozilla\Firefox\Extensions\Hello World"
When Firefox starts, it should load these plugins and you can just debug them in Firefox.
And Firefox should monitor these folders for changes and reload the plugins if necessary.