gecko-dev/dom/base/test/browser_pagehide_on_tab_close.js

function test() {
  waitForExplicitFinish();

  var tab = BrowserTestUtils.addTab(gBrowser);
  gBrowser.selectedTab = tab;

  tab.linkedBrowser.addEventListener(
    "load",
    function() {
      tab.linkedBrowser.addEventListener("pagehide", function() {
        ok(true, "got page hide event");
        finish();
      });

      executeSoon(() => {
        gBrowser.removeTab(tab);
      });
    },
    { capture: true, once: true }
  );
}