nsIDOMWindowUtils

README.md

nsIDOMWindowUtils provides some pretty interesting helpers. Here are some example chrome-privileged scripts.

animation-distance.js

// Get the 'distance' between 2 values of a CSS property

let win = gBrowser.mCurrentTab.linkedBrowser.contentWindow;
let utils = win.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowUtils)

let element = win.document.querySelector(".dot");
utils.computeAnimationDistance(element, "width", "10px", "11px");

control-time.js

// Disable the automatic refresh driver and control time manually

let win = gBrowser.mCurrentTab.linkedBrowser.contentWindow;
let utils = win.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowUtils)

utils.advanceTimeAndRefresh(0); // stop
utils.restoreNormalRefresh(); // restart
utils.advanceTimeAndRefresh(1000 / 60) // forward 1 tick
utils.advanceTimeAndRefresh(-1000 / 60) // backward 1 tick

get-framerate.js

// Get the framerate of paints on the current page, for a given amount of time

let {Task} = Cu.import("resource://gre/modules/Task.jsm", {});
let {Promise: promise} = Cu.import("resource://gre/modules/Promise.jsm", {});

let win = gBrowser.mCurrentTab.linkedBrowser.contentWindow;
let utils = win.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowUtils)

function wait(ms) {
  let def = promise.defer();
  win.setTimeout(def.resolve, ms);
  return def.promise;
}

// YOU HAVE 5 SECONDS TO PLAY WITH THE PAGE, AFTER THAT DELAY, THE FRAMERATE WILL BE DISPLAYED IN THE CONSOLE
let time = 5000;

Task.spawn(function*() {
  let recordingHandle = utils.startFrameTimeRecording();  
  yield wait(time);
  let intervals = utils.stopFrameTimeRecording(recordingHandle);
  let rate = intervals.length / (time/1000);
  win.console.log(rate);
});

set-resolution.js

// Set the resolution of the page (this doesn't reflow the page in any way)

let {Task} = Cu.import("resource://gre/modules/Task.jsm", {});
let {Promise: promise} = Cu.import("resource://gre/modules/Promise.jsm", {});

let win = gBrowser.mCurrentTab.linkedBrowser.contentWindow;
let utils = win.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowUtils)

function wait(ms) {
  let def = promise.defer();
  win.setTimeout(def.resolve, ms);
  return def.promise;
}

Task.spawn(function*() {
  for (let i = 1; i > 0; i -= .1) {
    utils.setResolution(i, i);
    yield wait(50);
  }
});

stop-animated-images.js

// Stop/Start all animated images in the window

let win = gBrowser.mCurrentTab.linkedBrowser.contentWindow;
let utils = win.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowUtils)
utils.imageAnimationMode = 0 // stop
utils.imageAnimationMode = 1 // start