--- a/browser/base/content/tabbrowser.xml
+++ b/browser/base/content/tabbrowser.xml
@@ -3427,16 +3427,23 @@
             startTabSwitch: function () {
               TelemetryStopwatch.cancel("FX_TAB_SWITCH_TOTAL_E10S_MS", window);
               TelemetryStopwatch.start("FX_TAB_SWITCH_TOTAL_E10S_MS", window);
               this.addMarker("AsyncTabSwitch:Start");
             },
 
             finishTabSwitch: function () {
               if (this.requestedTab && this.getTabState(this.requestedTab) == this.STATE_LOADED) {
+                // After this point the tab has switched from the content thread's point of view.
+                // The changes will be visible after the next refresh driver tick + composite.
+                let event = new CustomEvent("TabSwitched", {
+                  bubbles: true,
+                  cancelable: true
+                });
+                this.tabbrowser.dispatchEvent(event);
                 let time = TelemetryStopwatch.timeElapsed("FX_TAB_SWITCH_TOTAL_E10S_MS", window);
                 if (time != -1) {
                   TelemetryStopwatch.finish("FX_TAB_SWITCH_TOTAL_E10S_MS", window);
                   this.log("DEBUG: tab switch time = " + time);
                   this.addMarker("AsyncTabSwitch:Finish");
                 }
               }
             },