--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -259,16 +259,17 @@ nsString* nsContentUtils::sShiftText = n
 nsString* nsContentUtils::sControlText = nullptr;
 nsString* nsContentUtils::sMetaText = nullptr;
 nsString* nsContentUtils::sOSText = nullptr;
 nsString* nsContentUtils::sAltText = nullptr;
 nsString* nsContentUtils::sModifierSeparator = nullptr;
 
 bool nsContentUtils::sInitialized = false;
 bool nsContentUtils::sIsFullScreenApiEnabled = false;
+bool nsContentUtils::sIsUnprefixedFullscreenApiEnabled = false;
 bool nsContentUtils::sTrustedFullScreenOnly = true;
 bool nsContentUtils::sIsCutCopyAllowed = true;
 bool nsContentUtils::sIsFrameTimingPrefEnabled = false;
 bool nsContentUtils::sIsPerformanceTimingEnabled = false;
 bool nsContentUtils::sIsResourceTimingEnabled = false;
 bool nsContentUtils::sIsUserTimingLoggingEnabled = false;
 bool nsContentUtils::sIsExperimentalAutocompleteEnabled = false;
 bool nsContentUtils::sEncodeDecodeURLHash = false;
@@ -534,16 +535,19 @@ nsContentUtils::Init()
   sBlockedScriptRunners = new nsTArray< nsCOMPtr<nsIRunnable> >;
 
   Preferences::AddBoolVarCache(&sAllowXULXBL_for_file,
                                "dom.allow_XUL_XBL_for_file");
 
   Preferences::AddBoolVarCache(&sIsFullScreenApiEnabled,
                                "full-screen-api.enabled");
 
+  Preferences::AddBoolVarCache(&sIsUnprefixedFullscreenApiEnabled,
+                               "full-screen-api.unprefix.enabled");
+
   Preferences::AddBoolVarCache(&sTrustedFullScreenOnly,
                                "full-screen-api.allow-trusted-requests-only");
 
   Preferences::AddBoolVarCache(&sIsCutCopyAllowed,
                                "dom.allow_cut_copy", true);
 
   Preferences::AddBoolVarCache(&sIsPerformanceTimingEnabled,
                                "dom.enable_performance", true);

--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -1908,16 +1908,22 @@ public:
   static bool IsFocusedContent(const nsIContent *aContent);
 
   /**
    * Returns true if the DOM full-screen API is enabled.
    */
   static bool IsFullScreenApiEnabled();
 
   /**
+   * Returns true if the unprefixed fullscreen API is enabled.
+   */
+  static bool IsUnprefixedFullscreenApiEnabled()
+    { return sIsUnprefixedFullscreenApiEnabled; }
+
+  /**
    * Returns true if requests for full-screen are allowed in the current
    * context. Requests are only allowed if the user initiated them (like with
    * a mouse-click or key press), unless this check has been disabled by
    * setting the pref "full-screen-api.allow-trusted-requests-only" to false.
    */
   static bool IsRequestFullScreenAllowed();
 
   /**
@@ -2656,16 +2662,17 @@ private:
   static uint32_t sRunnersCountAtFirstBlocker;
   static uint32_t sScriptBlockerCountWhereRunnersPrevented;
 
   static nsIInterfaceRequestor* sSameOriginChecker;
 
   static bool sIsHandlingKeyBoardEvent;
   static bool sAllowXULXBL_for_file;
   static bool sIsFullScreenApiEnabled;
+  static bool sIsUnprefixedFullscreenApiEnabled;
   static bool sTrustedFullScreenOnly;
   static bool sIsCutCopyAllowed;
   static uint32_t sHandlingInputTimeout;
   static bool sIsPerformanceTimingEnabled;
   static bool sIsResourceTimingEnabled;
   static bool sIsUserTimingLoggingEnabled;
   static bool sIsFrameTimingPrefEnabled;
   static bool sIsExperimentalAutocompleteEnabled;

--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -11675,16 +11675,24 @@ nsresult nsDocument::RemoteFrameFullscre
 }
 
 nsresult nsDocument::RemoteFrameFullscreenReverted()
 {
   RestorePreviousFullScreenState();
   return NS_OK;
 }
 
+/* static */ bool
+nsDocument::IsUnprefixedFullscreenEnabled(JSContext* aCx, JSObject* aObject)
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  return nsContentUtils::IsCallerChrome() ||
+         nsContentUtils::IsUnprefixedFullscreenApiEnabled();
+}
+
 static void
 ReleaseVRDeviceProxyRef(void *, nsIAtom*, void *aPropertyValue, void *)
 {
   if (aPropertyValue) {
     static_cast<gfx::VRDeviceProxy*>(aPropertyValue)->Release();
   }
 }
 

--- a/dom/base/nsDocument.h
+++ b/dom/base/nsDocument.h
@@ -1231,16 +1231,18 @@ public:
   // Returns strong references to mBlockedTrackingNodes. (nsIDocument.h)
   //
   // This array contains nodes that have been blocked to prevent
   // user tracking. They most likely have had their nsIChannel
   // canceled by the URL classifier (Safebrowsing).
   //
   already_AddRefed<nsSimpleContentList> BlockedTrackingNodes() const;
 
+  static bool IsUnprefixedFullscreenEnabled(JSContext* aCx, JSObject* aObject);
+
   // Do the "fullscreen element ready check" from the fullscreen spec.
   // It returns true if the given element is allowed to go into fullscreen.
   bool FullscreenElementReadyCheck(Element* aElement, bool aWasCallerChrome);
 
   // This is called asynchronously by nsIDocument::AsyncRequestFullScreen()
   // to move this document into full-screen mode if allowed.
   void RequestFullScreen(mozilla::UniquePtr<FullscreenRequest>&& aRequest);
 

--- a/dom/webidl/Document.webidl
+++ b/dom/webidl/Document.webidl
@@ -219,23 +219,26 @@ partial interface Document {
   readonly attribute unsigned long referrerPolicy;
 
 };
 
 // https://fullscreen.spec.whatwg.org/#api
 partial interface Document {
   // Note: Per spec the 'S' in these two is lowercase, but the "Moz"
   // versions hve it uppercase.
+  [Func="nsDocument::IsUnprefixedFullscreenEnabled"]
   readonly attribute boolean fullscreenEnabled;
   [BinaryName="fullscreenEnabled", Deprecated="PrefixedFullscreenAPI"]
   readonly attribute boolean mozFullScreenEnabled;
+  [Func="nsDocument::IsUnprefixedFullscreenEnabled"]
   readonly attribute Element? fullscreenElement;
   [BinaryName="fullscreenElement", Deprecated="PrefixedFullscreenAPI"]
   readonly attribute Element? mozFullScreenElement;
 
+  [Func="nsDocument::IsUnprefixedFullscreenEnabled"]
   void exitFullscreen();
   [BinaryName="exitFullscreen", Deprecated="PrefixedFullscreenAPI"]
   void mozCancelFullScreen();
 
   // Gecko-specific fullscreen bits
   [Deprecated="PrefixedFullscreenAPI"]
   readonly attribute boolean mozFullScreen;
 };

--- a/dom/webidl/Element.webidl
+++ b/dom/webidl/Element.webidl
@@ -254,13 +254,13 @@ dictionary RequestFullscreenOptions {
 };
 
 // https://fullscreen.spec.whatwg.org/#api
 partial interface Element {
   /**
    * The options parameter is non-standard. In Gecko, it can be:
    *  a RequestFullscreenOptions object
    */
-  [Throws, UnsafeInPrerendering]
+  [Throws, UnsafeInPrerendering, Func="nsDocument::IsUnprefixedFullscreenEnabled"]
   void requestFullscreen(optional any options);
   [Throws, UnsafeInPrerendering, BinaryName="requestFullscreen", Deprecated="PrefixedFullscreenAPI"]
   void mozRequestFullScreen(optional any options);
 };

--- a/dom/webidl/EventHandler.webidl
+++ b/dom/webidl/EventHandler.webidl
@@ -113,17 +113,19 @@ interface GlobalEventHandlers {
            [Pref="dom.w3c_pointer_events.enabled"]
            attribute EventHandler onpointerleave;
            [Pref="dom.w3c_pointer_events.enabled"]
            attribute EventHandler ongotpointercapture;
            [Pref="dom.w3c_pointer_events.enabled"]
            attribute EventHandler onlostpointercapture;
 
            // Fullscreen events handlers
+           [Func="nsDocument::IsUnprefixedFullscreenEnabled"]
            attribute EventHandler onfullscreenchange;
+           [Func="nsDocument::IsUnprefixedFullscreenEnabled"]
            attribute EventHandler onfullscreenerror;
            attribute EventHandler onmozfullscreenchange;
            attribute EventHandler onmozfullscreenerror;
 
            // Mozilla-specific handlers
            attribute EventHandler onmozpointerlockchange;
            attribute EventHandler onmozpointerlockerror;
 };