--- a/testing/talos/talos/pageloader/chrome/tscroll.js
+++ b/testing/talos/talos/pageloader/chrome/tscroll.js
@@ -7,20 +7,73 @@ function testScroll(target, stepSize, op
   var win;
   if (target == "content") {
     target = content.wrappedJSObject;
     win = content;
   } else {
     win = window;
   }
 
+  var report;
+  /**
+   * Sets up the value of 'report' as a function for reporting the test result[s].
+   * Chooses between the "usual" tpRecordTime which the pageloader addon injects
+   * to pages, or a custom function in case we're a framescript which pageloader
+   * added to the tested page, or a debug tpRecordTime from talos-debug.js if
+   * running in a plain browser.
+   *
+   * @returns Promise
+   */
+  function P_setupReportFn() {
+    return new Promise(function(resolve) {
+      report = opt_reportFunc || win.tpRecordTime;
+      if (report == 'PageLoader:RecordTime') {
+        report = function(duration, start, name) {
+          var msg = { time: duration, startTime: start, testName: name };
+          sendAsyncMessage('PageLoader:RecordTime', msg);
+        }
+        resolve();
+        return;
+      }
+
+      // Not part of the test and does nothing if we're within talos.
+      // Provides an alternative tpRecordTime (with some stats display) if running in a browser.
+      if (!report && document.head) {
+        var imported = document.createElement('script');
+        imported.addEventListener("load", function() {
+          report = tpRecordTime;
+          resolve();
+        });
+
+        imported.src = '../../scripts/talos-debug.js?dummy=' + Date.now(); // For some browsers to re-read
+        document.head.appendChild(imported);
+        return;
+      }
+
+      resolve();
+    });
+  }
+
+  function FP_wait(ms) {
+    return function() {
+      return new Promise(function(resolve) {
+        setTimeout(resolve, ms);
+      });
+    };
+  }
+
   function rAF(fn) {
     return content.requestAnimationFrame(fn);
   }
 
+  function P_rAF() {
+    return new Promise(function(resolve) {
+      rAF(resolve);
+    });
+  }
 
   function myNow() {
     return (win.performance && win.performance.now) ?
             win.performance.now() :
             Date.now();
   };
 
   var isWindow = target.self === target;
@@ -39,89 +92,78 @@ function testScroll(target, stepSize, op
       this.dummyEnsureScroll = 1;
     }
     this.dummyEnsureScroll += win.screenY + getPos();
   }
 
   // For reference, rAF should fire on vsync, but Gecko currently doesn't use vsync.
   // Instead, it uses 1000/layout.frame_rate
   // (with 60 as default value when layout.frame_rate == -1).
-  function startTest()
-  {
-    // We should be at the top of the page now.
-    var start = myNow();
-    var lastScrollPos = getPos();
-    var lastScrollTime = start;
-    var durations = [];
-    var report = opt_reportFunc || tpRecordTime;
-    if (report == 'PageLoader:RecordTime') {
-      report = function(duration) {
-        var msg = { time: duration, startTime: '', testName: '' };
-        sendAsyncMessage('PageLoader:RecordTime', msg);
-      }
-    }
+  function P_syncScrollTest() {
+    return new Promise(function(resolve) {
+      // We should be at the top of the page now.
+      var start = myNow();
+      var lastScrollPos = getPos();
+      var lastScrollTime = start;
+      var durations = [];
+
+      function tick() {
+        var now = myNow();
+        var duration = now - lastScrollTime;
+        lastScrollTime = now;
+
+        durations.push(duration);
+        doScrollTick();
 
-    function tick() {
-      var now = myNow();
-      var duration = now - lastScrollTime;
-      lastScrollTime = now;
+        /* stop scrolling if we can't scroll more, or if we've reached requested number of steps */
+        if ((getPos() == lastScrollPos) || (opt_numSteps && (durations.length >= (opt_numSteps + 2)))) {
+          if (typeof(Profiler) !== "undefined") {
+            Profiler.pause();
+          }
+
+          // Note: The first (1-5) intervals WILL be longer than the rest.
+          // First interval might include initial rendering and be extra slow.
+          // Also requestAnimationFrame needs to sync (optimally in 1 frame) after long frames.
+          // Suggested: Ignore the first 5 intervals.
 
-      durations.push(duration);
-      doScrollTick();
+          durations.pop(); // Last step was 0.
+          durations.pop(); // and the prev one was shorter and with end-of-page logic, ignore both.
+
+          if (win.talosDebug)
+            win.talosDebug.displayData = true; // In a browser: also display all data points.
 
-      /* stop scrolling if we can't scroll more, or if we've reached requested number of steps */
-      if ((getPos() == lastScrollPos) || (opt_numSteps && (durations.length >= (opt_numSteps + 2)))) {
-        if (typeof(Profiler) !== "undefined") {
-          Profiler.pause();
+          // For analysis (otherwise, it's too many data points for talos):
+          var sum = 0;
+          for (var i = 0; i < durations.length; i++)
+            sum += Number(durations[i]);
+
+          // Report average interval or (failsafe) 0 if no intervls were recorded
+          resolve(durations.length ? sum / durations.length : 0);
+          return;
         }
 
-        // Note: The first (1-5) intervals WILL be longer than the rest.
-        // First interval might include initial rendering and be extra slow.
-        // Also requestAnimationFrame needs to sync (optimally in 1 frame) after long frames.
-        // Suggested: Ignore the first 5 intervals.
-
-        durations.pop(); // Last step was 0.
-        durations.pop(); // and the prev one was shorter and with end-of-page logic, ignore both.
-
-        if (win.talosDebug)
-          win.talosDebug.displayData = true; // In a browser: also display all data points.
-
-        // For analysis (otherwise, it's too many data points for talos):
-        var sum = 0;
-        for (var i = 0; i < durations.length; i++)
-          sum += Number(durations[i]);
-
-        // Report average interval or (failsafe) 0 if no intervls were recorded
-        report(durations.length ? sum / durations.length : 0);
-        return;
+        lastScrollPos = getPos();
+        rAF(tick);
       }
 
-      lastScrollPos = getPos();
+      if (typeof(Profiler) !== "undefined") {
+        Profiler.resume();
+      }
       rAF(tick);
-    }
-
-    if (typeof(Profiler) !== "undefined") {
-      Profiler.resume();
-    }
-    rAF(tick);
+    });
   }
 
-  // Not part of the test and does nothing if we're within talos,
-  // But provides an alternative tpRecordTime (with some stats display) if running in a browser
-  // If a callback is provided, then we don't need this debug reporting.
-  if(!opt_reportFunc && document.head) {
-    var imported = document.createElement('script');
-    imported.src = '../../scripts/talos-debug.js?dummy=' + Date.now(); // For some browsers to re-read
-    document.head.appendChild(imported);
-  }
-
-  setTimeout(function(){
-    gotoTop();
-    rAF(startTest);
-  }, 260);
+  P_setupReportFn()
+  .then(FP_wait(260))
+  .then(gotoTop)
+  .then(P_rAF)
+  .then(P_syncScrollTest)
+  .then(function(result) { // function since 'report' might not be ready when invoking 'then'
+    report(result);
+  });
 }
 
 // This code below here is unique to tscroll.js inside of pageloader
 try {
   function handleMessageFromChrome(message) {
     var payload = message.data.details;
     testScroll(payload.target, payload.stepSize, 'PageLoader:RecordTime', payload.opt_numSteps);
   }