Add firefox instructions to readme

README.md

@@ -45,6 +45,16 @@ Click `Select`.
 
 You now have the plugin, and can click 'inspect views: background plugin' to view its dev console.
 
+#### In Firefox (Developer Edition Only)
+
+Go to the url `about:debugging`.
+
+Click the button `Load Temporary Add-On`.
+
+Select the file `dist/manifest.json`.
+
+You can optionally enable debugging, and click `Debug`, for a console window that logs all of Metamask's processes to a single console.
+
 ### Developing on UI Only
 
 You can run `npm run ui`, and your browser should open a live-reloading demo version of the plugin UI.