gecko-dev/browser/base/content/test/browser_bug419612.js

function test() {
  waitForExplicitFinish();

  let testPage = "http://example.org/browser/browser/base/content/test/dummy_page.html";
  let tab1 = gBrowser.selectedTab = gBrowser.addTab();
  tab1.linkedBrowser.addEventListener("load", (function(event) {
    event.currentTarget.removeEventListener("load", arguments.callee, true);

    let tab2 = gBrowser.addTab();
    tab2.linkedBrowser.addEventListener("load", (function(event) {
      event.currentTarget.removeEventListener("load", arguments.callee, true);

      FullZoom.enlarge();
      let tab1Zoom = ZoomManager.getZoomForBrowser(tab1.linkedBrowser);

      afterZoom(function() {
        let tab2Zoom = ZoomManager.getZoomForBrowser(tab2.linkedBrowser);
        is(tab2Zoom, tab1Zoom, "Zoom should affect background tabs");

        gPrefService.setBoolPref("browser.zoom.updateBackgroundTabs", false);
        FullZoom.reset();
        gBrowser.selectedTab = tab1;
        tab1Zoom = ZoomManager.getZoomForBrowser(tab1.linkedBrowser);
        tab2Zoom = ZoomManager.getZoomForBrowser(tab2.linkedBrowser);
        isnot(tab1Zoom, tab2Zoom, "Zoom should not affect background tabs");

        if (gPrefService.prefHasUserValue("browser.zoom.updateBackgroundTabs"))
          gPrefService.clearUserPref("browser.zoom.updateBackgroundTabs");
        gBrowser.removeTab(tab1);
        gBrowser.removeTab(tab2);
        finish();
      });
      gBrowser.selectedTab = tab2;
    }), true);
    tab2.linkedBrowser.loadURI(testPage);
  }), true);
  content.location = testPage;
}

function afterZoom(cb) {
  let oldAPTS = FullZoom._applyPrefToSetting;
  FullZoom._applyPrefToSetting = function(value, browser) {
    if (!value)
      value = undefined;
    oldAPTS.call(FullZoom, value, browser);
    FullZoom._applyPrefToSetting = oldAPTS;
    executeSoon(cb);
  };
}