--- a/devtools/server/actors/animation.js
+++ b/devtools/server/actors/animation.js
@@ -385,24 +385,26 @@ var AnimationPlayerActor = protocol.Acto
         // Reset the local copy of the state on removal, since the animation can
         // be kept on the client and re-added, its state needs to be sent in
         // full.
         this.currentState = null;
       }
 
       if (hasCurrentAnimation(changedAnimations)) {
         // Only consider the state has having changed if any of delay, duration,
-        // iterationcount or iterationStart has changed (for now at least).
+        // iterationCount, iterationStart, or playbackRate has changed (for now
+        // at least).
         let newState = this.getState();
         let oldState = this.currentState;
         hasChanged = newState.delay !== oldState.delay ||
                      newState.iterationCount !== oldState.iterationCount ||
                      newState.iterationStart !== oldState.iterationStart ||
                      newState.duration !== oldState.duration ||
-                     newState.endDelay !== oldState.endDelay;
+                     newState.endDelay !== oldState.endDelay ||
+                     newState.playbackRate !== oldState.playbackRate;
         break;
       }
     }
 
     if (hasChanged) {
       this.emit("changed", this.getCurrentState());
     }
   },
@@ -459,17 +461,18 @@ var AnimationPlayerActor = protocol.Acto
     }
     this.player.currentTime = currentTime * this.player.playbackRate;
   },
 
   /**
    * Set the playback rate of the animation player.
    */
   setPlaybackRate: function (playbackRate) {
-    this.player.playbackRate = playbackRate;
+    this.player.updatePlaybackRate(playbackRate);
+    return this.player.ready;
   },
 
   /**
    * Get data about the keyframes of this animation player.
    * @return {Object} Returns a list of frames, each frame containing the list
    * animated properties as well as the frame's offset.
    */
   getFrames: function () {
@@ -865,13 +868,13 @@ exports.AnimationsActor = protocol.Actor
   },
 
   /**
    * Set the playback rate of several animations at the same time.
    * @param {Array} players A list of AnimationPlayerActor.
    * @param {Number} rate The new rate.
    */
   setPlaybackRates: function (players, rate) {
-    for (let player of players) {
-      player.setPlaybackRate(rate);
-    }
+    return Promise.all(
+      players.map(player => player.setPlaybackRate(rate))
+    );
   }
 });