Bug 41072: Set correct updater_url for Mullvad Browser nightly builds

projects/firefox/config

@@ -89,6 +89,7 @@ targets:
       branding_directory_prefix: 'mb'
       gitlab_project: https://gitlab.torproject.org/tpo/applications/mullvad-browser
       updater_url: 'https://cdn.mullvad.net/browser/update_responses/update_1/'
+      nightly_updates_publish_dir_prefix: mullvadbrowser-
 
   linux-x86_64:
     var: