--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -3092,16 +3092,22 @@ HTMLMediaElement::SetMuted(bool aMuted)
 
   DispatchAsyncEvent(NS_LITERAL_STRING("volumechange"));
 
   // We allow inaudible autoplay. But changing our mute status may make this
   // media audible. So pause if we are no longer supposed to be autoplaying.
   PauseIfShouldNotBePlaying();
 }
 
+double
+HTMLMediaElement::MozSeekStepLength() const
+{
+  return Preferences::GetInt("media.seekStepLength") /*ms*/ / 1000.0;
+}
+
 class HTMLMediaElement::StreamCaptureTrackSource
   : public MediaStreamTrackSource
   , public MediaStreamTrackSource::Sink
 {
 public:
   NS_DECL_ISUPPORTS_INHERITED
   NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(StreamCaptureTrackSource,
                                            MediaStreamTrackSource)

--- a/dom/html/HTMLMediaElement.h
+++ b/dom/html/HTMLMediaElement.h
@@ -637,16 +637,18 @@ public:
     return mIsCasting;
   }
 
   void SetMozIsCasting(bool aShow)
   {
     mIsCasting = aShow;
   }
 
+  double MozSeekStepLength() const;
+
   already_AddRefed<MediaSource> GetMozMediaSourceObject() const;
   // Returns a string describing the state of the media player internal
   // data. Used for debugging purposes.
   void GetMozDebugReaderData(nsAString& aString);
 
   // Returns a promise which will be resolved after collecting debugging
   // data from decoder/reader/MDSM. Used for debugging purposes.
   already_AddRefed<Promise> MozRequestDebugInfo(ErrorResult& aRv);

--- a/dom/webidl/HTMLMediaElement.webidl
+++ b/dom/webidl/HTMLMediaElement.webidl
@@ -115,18 +115,21 @@ partial interface HTMLMediaElement {
   [Pref="media.test.dumpDebugInfo"]
   Promise<void> mozDumpDebugInfo();
 
   attribute MediaStream? srcObject;
 
   attribute boolean mozPreservesPitch;
 
   // NB: for internal use with the video controls:
-  [Func="IsChromeOrXBL"] attribute boolean mozAllowCasting;
-  [Func="IsChromeOrXBL"] attribute boolean mozIsCasting;
+  [Func="IsChromeOrXBL"]
+  attribute boolean mozAllowCasting;
+  [Func="IsChromeOrXBL"]
+  attribute boolean mozIsCasting;
+  readonly attribute double mozSeekStepLength;
 
   // Mozilla extension: stream capture
   [Throws]
   MediaStream mozCaptureStream();
   [Throws]
   MediaStream mozCaptureStreamUntilEnded();
   readonly attribute boolean mozAudioCaptured;
 

--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -5718,16 +5718,20 @@ pref("layout.css.color-adjust.enabled", 
 pref("dom.audiochannel.audioCompeting", false);
 pref("dom.audiochannel.audioCompeting.allAgents", false);
 
 // Default media volume
 pref("media.default_volume", "1.0");
 
 pref("media.seekToNextFrame.enabled", true);
 
+// The amount of milliseconds to seek backward or forward when using a fixed
+// (non-relative) seek step.
+pref("media.seekStepLength", 15000);
+
 // return the maximum number of cores that navigator.hardwareCurrency returns
 pref("dom.maxHardwareConcurrency", 16);
 
 // Shutdown the osfile worker if its no longer needed.
 #if !defined(RELEASE_OR_BETA)
 pref("osfile.reset_worker_delay", 30000);
 #endif
 

--- a/toolkit/content/widgets/videocontrols.xml
+++ b/toolkit/content/widgets/videocontrols.xml
@@ -1433,17 +1433,17 @@
       },
 
       keyHandler(event) {
         // Ignore keys when content might be providing its own.
         if (!this.video.hasAttribute("controls")) {
           return;
         }
 
-        var keystroke = "";
+        let keystroke = "";
         if (event.altKey) {
           keystroke += "alt-";
         }
         if (event.shiftKey) {
           keystroke += "shift-";
         }
         if (navigator.platform.startsWith("Mac")) {
           if (event.metaKey) {
@@ -1481,17 +1481,17 @@
             break;
         }
 
         if (String.fromCharCode(event.charCode) == " ") {
           keystroke += "space";
         }
 
         this.log("Got keystroke: " + keystroke);
-        var oldval, newval;
+        let oldval, newval;
 
         try {
           switch (keystroke) {
             case "space": /* Play */
               let target = event.originalTarget;
               if (target.localName === "button" && !target.disabled) {
                 break;
               }
@@ -1509,43 +1509,37 @@
               this.video.muted = false;
               break;
             case "accel-downArrow": /* Mute */
               this.video.muted = true;
               break;
             case "accel-upArrow": /* Unmute */
               this.video.muted = false;
               break;
-            case "leftArrow": /* Seek back 15 seconds */
-            case "accel-leftArrow": /* Seek back 10% */
-              oldval = this.video.currentTime;
-              if (keystroke == "leftArrow") {
-                newval = oldval - 15;
-              } else {
-                newval = oldval - (this.video.duration || this.maxCurrentTimeSeen / 1000) / 10;
-              }
-              this.video.currentTime = (newval >= 0 ? newval : 0);
+            case "leftArrow":          /* Seek backward...             */
+            case "rightArrow":         /* ...or forward a fixed amount */
+            case "accel-leftArrow":    /* Seek backward...             */
+            case "accel-rightArrow": { /* ...or forward 10%            */
+              let maxTime = this.video.duration /*s*/ ||
+                            this.maxCurrentTimeSeen /*ms*/ / 1000;
+              let seekStepLength = keystroke.startsWith("accel-") ?
+                  maxTime * 0.10
+                : this.video.mozSeekStepLength /*s*/;
+              this.video.currentTime = keystroke.endsWith("leftArrow") ?
+                  Math.max(this.video.currentTime - seekStepLength, 0)       // <-
+                : Math.min(this.video.currentTime + seekStepLength, maxTime) // ->
               break;
-            case "rightArrow": /* Seek forward 15 seconds */
-            case "accel-rightArrow": /* Seek forward 10% */
-              oldval = this.video.currentTime;
-              var maxtime = (this.video.duration || this.maxCurrentTimeSeen / 1000);
-              if (keystroke == "rightArrow") {
-                newval = oldval + 15;
-              } else {
-                newval = oldval + maxtime / 10;
-              }
-              this.video.currentTime = (newval <= maxtime ? newval : maxtime);
-              break;
+            }
             case "home": /* Seek to beginning */
               this.video.currentTime = 0;
               break;
             case "end": /* Seek to end */
-              if (this.video.currentTime != this.video.duration) {
-                this.video.currentTime = (this.video.duration || this.maxCurrentTimeSeen / 1000);
+              if (this.video.currentTime /*s*/ != this.video.duration /*s*/) {
+                this.video.currentTime = this.video.duration /*s*/ ||
+                                         this.maxCurrentTimeSeen /*ms*/ / 1000;
               }
               break;
             default:
               return;
           }
         } catch (e) { /* ignore any exception from setting .currentTime */ }
 
         event.preventDefault(); // Prevent page scrolling