--- a/testing/marionette/browser.js
+++ b/testing/marionette/browser.js
@@ -156,16 +156,31 @@ browser.Context = class {
       return this.contentBrowser.currentURI.spec;
 
     } else {
       throw new NoSuchWindowError("Current window does not have a content browser");
     }
   }
 
   /**
+   * Gets the position and dimensions of the top-level browsing context.
+   *
+   * @return {Map.<string, number>}
+   *     Object with |x|, |y|, |width|, and |height| properties.
+   */
+  get rect() {
+    return {
+      x: this.window.screenX,
+      y: this.window.screenY,
+      width: this.window.outerWidth,
+      height: this.window.outerHeight,
+    };
+  }
+
+  /**
    * Retrieves the current tabmodal UI object.  According to the browser
    * associated with the currently selected tab.
    */
   getTabModalUI() {
     let br = this.contentBrowser;
     if (!br.hasAttribute("tabmodalPromptShowing")) {
       return null;
     }

--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -1257,22 +1257,17 @@ GeckoDriver.prototype.getChromeWindowHan
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
 GeckoDriver.prototype.getWindowRect = function (cmd, resp) {
   const win = assert.window(this.getCurrentWindow());
   assert.noUserPrompt(this.dialog);
 
-  return {
-    x: win.screenX,
-    y: win.screenY,
-    width: win.outerWidth,
-    height: win.outerHeight,
-  };
+  return this.curBrowser.rect;
 };
 
 /**
  * Set the window position and size of the browser on the operating
  * system window manager.
  *
  * The supplied |width| and |height| values refer to the window outerWidth
  * and outerHeight values, which include browser chrome and OS-level
@@ -1295,65 +1290,98 @@ GeckoDriver.prototype.getWindowRect = fu
  *
  * @throws {UnsupportedOperationError}
  *     Not applicable to application.
  * @throws {NoSuchWindowError}
  *     Top-level browsing context has been discarded.
  * @throws {UnexpectedAlertOpenError}
  *     A modal dialog is open, blocking this operation.
  */
-GeckoDriver.prototype.setWindowRect = function* (cmd, resp) {
+GeckoDriver.prototype.setWindowRect = async function (cmd, resp) {
   assert.firefox()
   const win = assert.window(this.getCurrentWindow());
   assert.noUserPrompt(this.dialog);
 
   let {x, y, width, height} = cmd.parameters;
-  let optimisedResize = resolve => () => win.requestAnimationFrame(resolve);
-
-  if (win.windowState == win.STATE_FULLSCREEN) {
-    yield new Promise(resolve => {
+  let origRect = this.curBrowser.rect;
+
+  // Throttle resize event by forcing the event queue to flush and delay
+  // until the main thread is idle.
+  function optimisedResize (resolve) {
+    return () => Services.tm.mainThread.idleDispatch(() => {
+      win.requestAnimationFrame(resolve);
+    });
+  }
+
+  // Exit fullscreen and wait for window to resize.
+  async function exitFullscreen () {
+    return new Promise(resolve => {
       win.addEventListener("sizemodechange", optimisedResize(resolve), {once: true});
       win.fullScreen = false;
     });
   }
 
+  // Synchronous resize to |width| and |height| dimensions.
+  async function resizeWindow (width, height) {
+    return new Promise(resolve => {
+      win.addEventListener("resize", optimisedResize(resolve), {once: true});
+      win.resizeTo(width, height);
+    });
+  }
+
+  // Wait until window size has changed.  We can't wait for the
+  // user-requested window size as this may not be achievable on the
+  // current system.
+  const windowResizeChange = async () => {
+    return wait.until((resolve, reject) => {
+      let curRect = this.curBrowser.rect;
+      if (curRect.width != origRect.width &&
+          curRect.height != origRect.height) {
+        resolve();
+      } else {
+        reject();
+      }
+    });
+  };
+
+  // Wait for the window position to change.
+  async function windowPosition (x, y) {
+    return wait.until((resolve, reject) => {
+      if ((x == win.screenX && y == win.screenY) ||
+          (win.screenX != origRect.x || win.screenY != origRect.y)) {
+        resolve();
+      } else {
+        reject();
+      }
+    });
+  }
+
+  if (win.windowState == win.STATE_FULLSCREEN) {
+    await exitFullscreen();
+  }
+
   if (height != null && width != null) {
     assert.positiveInteger(height);
     assert.positiveInteger(width);
 
     if (win.outerWidth != width || win.outerHeight != height) {
-      yield new Promise(resolve => {
-        win.addEventListener("resize", optimisedResize(resolve), {once: true});
-        win.resizeTo(width, height);
-      });
+      await resizeWindow(width, height);
+      await windowResizeChange();
     }
   }
 
   if (x != null && y != null) {
     assert.integer(x);
     assert.integer(y);
-    const orig = {screenX: win.screenX, screenY: win.screenY};
 
     win.moveTo(x, y);
-    yield wait.until((resolve, reject) => {
-      if ((x == win.screenX && y == win.screenY) ||
-        (win.screenX != orig.screenX || win.screenY != orig.screenY)) {
-        resolve();
-      } else {
-        reject();
-      }
-    });
+    await windowPosition(x, y);
   }
 
-  resp.body = {
-    "x": win.screenX,
-    "y": win.screenY,
-    "width": win.outerWidth,
-    "height": win.outerHeight,
-  };
+  return this.curBrowser.rect;
 };
 
 /**
  * Switch current top-level browsing context by name or server-assigned ID.
  * Searches for windows by name, then ID.  Content windows take precedence.
  *
  * @param {string} name
  *     Target name or ID of the window to switch to.