Substitute about:debugging#/runtime/this-firefox for about:debugging (#…

…488)

* Substitute about:debugging#/runtime/this-firefox for about:debugging

Extensions are loaded at about:debugging#/runtime/this-firefox not about:debugging

* Drop Python 2 support. Fixes #489

* Update README.md

* Revert to ae1c217

* Update native-messaging/README.md

Co-authored-by: rebloor <git@sherpa.co.nz>

---------

Co-authored-by: rebloor <git@sherpa.co.nz>

native-messaging/README.md

@@ -25,7 +25,7 @@ To assist in troubleshooting on Windows, there is a script called `check_config_
 
 ## Testing the example ##
 
-Then just install the add-on as usual, by visiting about:debugging, clicking "Load Temporary Add-on", and selecting the add-on's "manifest.json".
+First, install the add-on. Visit `about:debugging#/runtime/this-firefox` or, from `about:debugging` click "This Firefox" (or "This Nightly" in the Nightly version of Firefox), click "Load Temporary Add-on", and open the add-on's "manifest.json".
 
 Now, open the extension's console using the "Inspect" button - this is where you'll see communication between the browser and native app.