fune/layout/base/tests/browser_disableDialogs_onbeforeunload.js

function pageScript() {
  window.addEventListener(
    "beforeunload",
    function(event) {
      var str = "Some text that causes the beforeunload dialog to be shown";
      event.returnValue = str;
      return str;
    },
    true
  );
}

SpecialPowers.pushPrefEnv({
  set: [["dom.require_user_interaction_for_beforeunload", false]],
});

const PAGE_URL =
  "data:text/html," +
  encodeURIComponent("<script>(" + pageScript.toSource() + ")();</script>");

add_task(async function enableDialogs() {
  // The onbeforeunload dialog should appear.
  let dialogShown = false;
  function onDialogShown(node) {
    dialogShown = true;
    let dismissButton = node.querySelector(".tabmodalprompt-button0");
    dismissButton.click();
  }
  let obsName = "tabmodal-dialog-loaded";
  Services.obs.addObserver(onDialogShown, obsName);
  await openPage(true);
  Services.obs.removeObserver(onDialogShown, obsName);
  Assert.ok(dialogShown);
});

add_task(async function disableDialogs() {
  // The onbeforeunload dialog should NOT appear.
  await openPage(false);
  info("If we time out here, then the dialog was shown...");
});

async function openPage(enableDialogs) {
  // Open about:blank in a new tab.
  await BrowserTestUtils.withNewTab(
    { gBrowser, url: "about:blank" },
    async function(browser) {
      // Load the content script in the frame.
      let methodName = enableDialogs ? "enableDialogs" : "disableDialogs";
      await ContentTask.spawn(browser, methodName, async function(name) {
        const { Services } = ChromeUtils.import(
          "resource://gre/modules/Services.jsm"
        );
        Services.obs.addObserver(doc => {
          if (content && doc == content.document) {
            content.windowUtils[name]();
          }
        }, "document-element-inserted");
      });
      // Load the page.
      await BrowserTestUtils.loadURI(browser, PAGE_URL);
      await BrowserTestUtils.browserLoaded(browser);
      // And then navigate away.
      await BrowserTestUtils.loadURI(browser, "http://example.com/");
      await BrowserTestUtils.browserLoaded(browser);
    }
  );
}