--- a/testing/geckodriver/src/marionette.rs
+++ b/testing/geckodriver/src/marionette.rs
@@ -483,23 +483,20 @@ impl MarionetteHandler {
         for &(ref name, ref value) in prefs::DEFAULT.iter() {
             if !custom_profile || !prefs.contains_key(name) {
                 prefs.insert((*name).clone(), (*value).clone());
             }
         }
 
         prefs.insert_slice(&extra_prefs[..]);
 
-        // fallbacks can be removed when Firefox 54 becomes stable
         if let Some(ref level) = self.current_log_level {
             prefs.insert("marionette.log.level", Pref::new(level.to_string()));
-            prefs.insert("marionette.logging", Pref::new(level.to_string()));  // fallback
         };
         prefs.insert("marionette.port", Pref::new(port as i64));
-        prefs.insert("marionette.defaultPrefs.port", Pref::new(port as i64));  // fallback
 
         prefs.write().map_err(|_| WebDriverError::new(ErrorStatus::UnknownError,
                                                       "Unable to write Firefox profile"))
     }
 }
 
 impl WebDriverHandler<GeckoExtensionRoute> for MarionetteHandler {
     fn handle_command(&mut self, _: &Option<Session>,