--- a/browser/modules/ProcessHangMonitor.jsm
+++ b/browser/modules/ProcessHangMonitor.jsm
@@ -391,31 +391,41 @@ var ProcessHangMonitor = {
 
   /**
    * Install event handlers on |win| to watch for events that would
    * cause a different hang report to be displayed.
    */
   trackWindow(win) {
     win.gBrowser.tabContainer.addEventListener("TabSelect", this, true);
     win.gBrowser.tabContainer.addEventListener("TabRemotenessChange", this, true);
+    win.addEventListener("unload", this, true);
   },
 
   untrackWindow(win) {
     win.gBrowser.tabContainer.removeEventListener("TabSelect", this, true);
     win.gBrowser.tabContainer.removeEventListener("TabRemotenessChange", this, true);
+    win.removeEventListener("unload", this, true);
   },
 
   handleEvent(event) {
     let win = event.target.ownerGlobal;
 
-    // If a new tab is selected or if a tab changes remoteness, then
-    // we may need to show or hide a hang notification.
-
-    if (event.type == "TabSelect" || event.type == "TabRemotenessChange") {
-      this.updateWindow(win);
+    switch (event.type) {
+      // If a new tab is selected or if a tab changes remoteness, then
+      // we may need to show or hide a hang notification.
+      case "TabSelect":
+        // intentional fall-through
+      case "TabRemotenessChange": {
+        this.updateWindow(win);
+        break;
+      }
+      case "unload": {
+        this.untrackWindow(win);
+        break;
+      }
     }
   },
 
   /**
    * Handle a potentially new hang report. If it hasn't been seen
    * before, show a notification for it in all open XUL windows.
    */
   reportHang(report) {