--- a/browser/modules/AsyncTabSwitcher.jsm
+++ b/browser/modules/AsyncTabSwitcher.jsm
@@ -1028,16 +1028,19 @@ class AsyncTabSwitcher {
     TelemetryStopwatch.start("FX_TAB_SWITCH_SPINNER_VISIBLE_MS", this.window);
     // We have a second, similar probe for capturing recordings of
     // when the spinner is displayed for very long periods.
     TelemetryStopwatch.start("FX_TAB_SWITCH_SPINNER_VISIBLE_LONG_MS", this.window);
     this.addMarker("AsyncTabSwitch:SpinnerShown");
     Services.telemetry
       .getHistogramById("FX_TAB_SWITCH_SPINNER_VISIBLE_TRIGGER")
       .add(this._loadTimerClearedBy);
+    if (AppConstants.NIGHTLY_BUILD) {
+      Services.obs.notifyObservers(null, "tabswitch-spinner");
+    }
   }
 
   spinnerHidden() {
     this.assert(this.spinnerTab);
     this.log("DEBUG: spinner time = " +
       TelemetryStopwatch.timeElapsed("FX_TAB_SWITCH_SPINNER_VISIBLE_MS", this.window));
     TelemetryStopwatch.finish("FX_TAB_SWITCH_SPINNER_VISIBLE_MS", this.window);
     TelemetryStopwatch.finish("FX_TAB_SWITCH_SPINNER_VISIBLE_LONG_MS", this.window);