Read Firefox Extension ID from appropriate location in manifest.json

Previously, when installing an extension that didn't have an install.rdf
file and instead relied on using the manifest.json, we weren't looking
in the correct place for the extensions gecko ID.

https://extensionworkshop.com/documentation/develop/extensions-and-the-add-on-id/#When_do_you_need_an_add-on_ID

If someone specifies this ID themselves, we need to check the
manifest.json for browser_specific_settings, which then houses the
application ID. Otherwise, we can fallback to using the extension name +
version.

rb/lib/selenium/webdriver/firefox/extension.rb

@@ -87,6 +87,14 @@ def read_id_from_manifest_json(directory)
           return unless File.exist?(manifest_path)
 
           manifest = JSON.parse(File.read(manifest_path))
+          gecko_id(manifest) || name_and_version(manifest)
+        end
+
+        def gecko_id(manifest)
+          manifest.dig('browser_specific_settings', 'gecko', 'id')&.strip
+        end
+
+        def name_and_version(manifest)
           [manifest['name'].delete(' '), manifest['version']].join('@')
         end
       end # Extension