diff --git a/testing/profiles/prefs_general.js b/testing/profiles/prefs_general.js
index d168dfb739b2..651c8696a265 100644
--- a/testing/profiles/prefs_general.js
+++ b/testing/profiles/prefs_general.js
@@ -41,7 +41,7 @@ user_pref("security.warn_viewing_mixed", false);
 user_pref("app.update.enabled", false);
 user_pref("app.update.staging.enabled", false);
 // Make sure GMPInstallManager won't hit the network.
-user_pref("media.gmp-manager.url.override", "http://%(server)s/dummy.xml");
+user_pref("media.gmp-manager.url.override", "http://%(server)s/dummy-gmp-manager.xml");
 user_pref("browser.panorama.experienced_first_run", true); // Assume experienced
 user_pref("dom.w3c_touch_events.enabled", 1);
 user_pref("dom.undo_manager.enabled", true);
diff --git a/testing/xpcshell/head.js b/testing/xpcshell/head.js
index 0f7232fa5bf8..6e5707d881b2 100644
--- a/testing/xpcshell/head.js
+++ b/testing/xpcshell/head.js
@@ -1349,6 +1349,6 @@ try {
     let prefs = Components.classes["@mozilla.org/preferences-service;1"]
       .getService(Components.interfaces.nsIPrefBranch);
 
-    prefs.setCharPref("media.gmp-manager.url.override", "http://%(server)s/dummy.xml");
+    prefs.setCharPref("media.gmp-manager.url.override", "http://%(server)s/dummy-gmp-manager.xml");
   }
 } catch (e) { }