--- a/browser/devtools/animationinspector/test/head.js
+++ b/browser/devtools/animationinspector/test/head.js
@@ -154,17 +154,22 @@ let openAnimationInspector = Task.async(
   inspector.sidebar.select("animationinspector");
 
   info("Waiting for the inspector and sidebar to be ready");
   yield promise.all(initPromises);
 
   let win = inspector.sidebar.getWindowForTab("animationinspector");
   let {AnimationsController, AnimationsPanel} = win;
 
-  yield AnimationsPanel.once(AnimationsPanel.PANEL_INITIALIZED);
+  info("Waiting for the animation controller and panel to be ready");
+  if (AnimationsPanel.initialized) {
+    yield AnimationsPanel.initialized;
+  } else {
+    yield AnimationsPanel.once(AnimationsPanel.PANEL_INITIALIZED);
+  }
 
   return {
     toolbox: toolbox,
     inspector: inspector,
     controller: AnimationsController,
     panel: AnimationsPanel,
     window: win
   };