--- a/widget/gtk/mozgtk/mozgtk.c
+++ b/widget/gtk/mozgtk/mozgtk.c
@@ -54,16 +54,17 @@ STUB(gdk_property_get)
 STUB(gdk_screen_get_default)
 STUB(gdk_screen_get_display)
 STUB(gdk_screen_get_font_options)
 STUB(gdk_screen_get_height)
 STUB(gdk_screen_get_height_mm)
 STUB(gdk_screen_get_monitor_at_window)
 STUB(gdk_screen_get_monitor_geometry)
 STUB(gdk_screen_get_number)
+STUB(gdk_screen_get_n_monitors)
 STUB(gdk_screen_get_resolution)
 STUB(gdk_screen_get_rgba_visual)
 STUB(gdk_screen_get_root_window)
 STUB(gdk_screen_get_system_visual)
 STUB(gdk_screen_get_width)
 STUB(gdk_screen_height)
 STUB(gdk_screen_is_composited)
 STUB(gdk_screen_width)
@@ -509,16 +510,18 @@ STUB(gtk_window_unmaximize)
 #ifdef GTK3_SYMBOLS
 STUB(gdk_device_get_source)
 STUB(gdk_device_manager_get_client_pointer)
 STUB(gdk_disable_multidevice)
 STUB(gdk_device_manager_list_devices)
 STUB(gdk_display_get_device_manager)
 STUB(gdk_error_trap_pop_ignored)
 STUB(gdk_event_get_source_device)
+STUB(gdk_screen_get_monitor_workarea)
+STUB(gdk_screen_get_primary_monitor)
 STUB(gdk_window_get_type)
 STUB(gdk_x11_window_get_xid)
 STUB(gdk_x11_display_get_type)
 STUB(gtk_cairo_should_draw_window)
 STUB(gtk_cairo_transform_to_window)
 STUB(gtk_combo_box_text_append)
 STUB(gtk_drag_set_icon_surface)
 STUB(gtk_get_major_version)

--- a/widget/gtk/nsScreenGtk.cpp
+++ b/widget/gtk/nsScreenGtk.cpp
@@ -13,18 +13,17 @@
 #include <gtk/gtk.h>
 #include <dlfcn.h>
 #include "gfxPlatformGtk.h"
 
 static uint32_t sScreenId = 0;
 
 
 nsScreenGtk :: nsScreenGtk (  )
-  : mScreenNum(0),
-    mRect(0, 0, 0, 0),
+  : mRect(0, 0, 0, 0),
     mAvailRect(0, 0, 0, 0),
     mId(++sScreenId)
 {
 }
 
 
 nsScreenGtk :: ~nsScreenGtk()
 {
@@ -60,27 +59,27 @@ nsScreenGtk :: GetAvailRect(int32_t *out
   *outWidth = mAvailRect.width;
   *outHeight = mAvailRect.height;
 
   return NS_OK;
   
 } // GetAvailRect
 
 gint
-nsScreenGtk :: GetGtkMonitorScaleFactor()
+nsScreenGtk :: GetGtkMonitorScaleFactor(gint aNum)
 {
 #if (MOZ_WIDGET_GTK >= 3)
   // Since GDK 3.10
   static auto sGdkScreenGetMonitorScaleFactorPtr = (gint (*)(GdkScreen*, gint))
       dlsym(RTLD_DEFAULT, "gdk_screen_get_monitor_scale_factor");
   if (sGdkScreenGetMonitorScaleFactorPtr) {
       // FIXME: In the future, we'll want to fix this for GTK on Wayland which
       // supports a variable scale factor per display.
       GdkScreen *screen = gdk_screen_get_default();
-      return sGdkScreenGetMonitorScaleFactorPtr(screen, 0);
+      return sGdkScreenGetMonitorScaleFactorPtr(screen, aNum);
   }
 #endif
     return 1;
 }
 
 double
 nsScreenGtk :: GetDPIScale()
 {
@@ -186,20 +185,33 @@ nsScreenGtk :: Init (GdkWindow *aRootWin
 
       mAvailRect.IntersectRect(mAvailRect, workarea);
     }
   }
   g_free (workareas);
 #endif
 }
 
-#ifdef MOZ_X11
 void
-nsScreenGtk :: Init (XineramaScreenInfo *aScreenInfo)
+nsScreenGtk :: Init (GdkScreen *aScreen, gint aIndex, gint aTotal)
 {
-  nsIntRect xineRect(aScreenInfo->x_org, aScreenInfo->y_org,
-                     aScreenInfo->width, aScreenInfo->height);
+#if (MOZ_WIDGET_GTK == 2)
+  // GTK 3.4 early doesn't have gdk_screen_get_monitor_workarea
+  if (aTotal == 1) {
+    Init(gdk_screen_get_root_window(aScreen));
+    return;
+  }
+#endif
 
-  mScreenNum = aScreenInfo->screen_number;
+  GdkRectangle rect;
+  gint scale = nsScreenGtk::GetGtkMonitorScaleFactor(aIndex);
+  gdk_screen_get_monitor_geometry(aScreen, aIndex, &rect);
+  mRect = nsIntRect(rect.x * scale, rect.y * scale,
+                    rect.width * scale, rect.height * scale);
 
-  mAvailRect = mRect = xineRect;
+#if (MOZ_WIDGET_GTK == 3)
+  gdk_screen_get_monitor_workarea(aScreen, aIndex, &rect);
+  mAvailRect = nsIntRect(rect.x * scale, rect.y * scale,
+                         rect.width * scale, rect.height * scale);
+#else
+  mAvailRect = mRect;
+#endif
 }
-#endif

--- a/widget/gtk/nsScreenGtk.h
+++ b/widget/gtk/nsScreenGtk.h
@@ -4,28 +4,16 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #ifndef nsScreenGtk_h___
 #define nsScreenGtk_h___
 
 #include "nsBaseScreen.h"
 #include "nsRect.h"
 #include "gdk/gdk.h"
-#ifdef MOZ_X11
-#include <X11/Xlib.h>
-
-// from Xinerama.h
-typedef struct {
-   int   screen_number;
-   short x_org;
-   short y_org;
-   short width;
-   short height;
-} XineramaScreenInfo;
-#endif /* MOZ_X11 */
 
 //------------------------------------------------------------------------
 
 class nsScreenGtk : public nsBaseScreen
 {
 public:
   nsScreenGtk();
   ~nsScreenGtk();
@@ -34,24 +22,22 @@ public:
   NS_IMETHOD GetRect(int32_t* aLeft, int32_t* aTop,
                      int32_t* aWidth, int32_t* aHeight) override;
   NS_IMETHOD GetAvailRect(int32_t* aLeft, int32_t* aTop,
                           int32_t* aWidth, int32_t* aHeight) override;
   NS_IMETHOD GetPixelDepth(int32_t* aPixelDepth) override;
   NS_IMETHOD GetColorDepth(int32_t* aColorDepth) override;
   NS_IMETHOD GetDefaultCSSScaleFactor(double* aScaleFactor) override;
 
-  void Init(GdkWindow *aRootWindow);
-#ifdef MOZ_X11
-  void Init(XineramaScreenInfo *aScreenInfo);
-#endif /* MOZ_X11 */
+  void Init(GdkScreen *aScreen, gint aIndex, gint aTotal);
 
-  static gint    GetGtkMonitorScaleFactor();
+  static gint    GetGtkMonitorScaleFactor(gint aNum = 0);
   static double  GetDPIScale();
 
 private:
-  uint32_t mScreenNum;
+  void Init(GdkWindow *aRootWindow);
+
   nsIntRect mRect;
   nsIntRect mAvailRect;
   uint32_t mId;
 };
 
 #endif  // nsScreenGtk_h___

--- a/widget/gtk/nsScreenManagerGtk.cpp
+++ b/widget/gtk/nsScreenManagerGtk.cpp
@@ -5,23 +5,18 @@
 
 #include "nsScreenManagerGtk.h"
 #include "nsScreenGtk.h"
 #include "nsIComponentManager.h"
 #include "nsRect.h"
 #include "nsAutoPtr.h"
 #include "nsGtkUtils.h"
 
-#define SCREEN_MANAGER_LIBRARY_LOAD_FAILED ((PRLibrary*)1)
-
 #ifdef MOZ_X11
 #include <gdk/gdkx.h>
-// prototypes from Xinerama.h
-typedef Bool (*_XnrmIsActive_fn)(Display *dpy);
-typedef XineramaScreenInfo* (*_XnrmQueryScreens_fn)(Display *dpy, int *number);
 #endif
 
 #include <gtk/gtk.h>
 
 void
 monitors_changed(GdkScreen* aScreen, gpointer aClosure)
 {
   nsScreenManagerGtk *manager = static_cast<nsScreenManagerGtk*>(aClosure);
@@ -50,19 +45,20 @@ root_window_event_filter(GdkXEvent *aGdk
       break;
   }
 #endif
 
   return GDK_FILTER_CONTINUE;
 }
 
 nsScreenManagerGtk :: nsScreenManagerGtk ( )
-  : mXineramalib(nullptr)
-  , mRootWindow(nullptr)
+  : mRootWindow(nullptr)
+#ifdef MOZ_X11
   , mNetWorkareaAtom(0)
+#endif
 {
   // nothing else to do. I guess we could cache a bunch of information
   // here, but we want to ask the device at runtime in case anything
   // has changed.
 }
 
 
 nsScreenManagerGtk :: ~nsScreenManagerGtk()
@@ -121,93 +117,36 @@ nsScreenManagerGtk :: EnsureInit()
 #endif
 
   return Init();
 }
 
 nsresult
 nsScreenManagerGtk :: Init()
 {
-#ifdef MOZ_X11
-  XineramaScreenInfo *screenInfo = nullptr;
-  int numScreens;
+  GdkScreen* gdkScreen = gdk_screen_get_default();
 
-  bool useXinerama = GDK_IS_X11_DISPLAY(gdk_display_get_default());
-
-  if (useXinerama && !mXineramalib) {
-    mXineramalib = PR_LoadLibrary("libXinerama.so.1");
-    if (!mXineramalib) {
-      mXineramalib = SCREEN_MANAGER_LIBRARY_LOAD_FAILED;
+  gint numScreens = gdk_screen_get_n_monitors(gdkScreen);
+  for (gint i = 0; i < numScreens; i++) {
+    RefPtr<nsScreenGtk> screen;
+    if (mCachedScreenArray.Count() > i) {
+      screen = static_cast<nsScreenGtk*>(mCachedScreenArray[i]);
+    } else {
+      screen = new nsScreenGtk();
+      mCachedScreenArray.AppendObject(screen);
     }
-  }
-  if (mXineramalib && mXineramalib != SCREEN_MANAGER_LIBRARY_LOAD_FAILED) {
-    _XnrmIsActive_fn _XnrmIsActive = (_XnrmIsActive_fn)
-        PR_FindFunctionSymbol(mXineramalib, "XineramaIsActive");
-
-    _XnrmQueryScreens_fn _XnrmQueryScreens = (_XnrmQueryScreens_fn)
-        PR_FindFunctionSymbol(mXineramalib, "XineramaQueryScreens");
-        
-    // get the number of screens via xinerama
-    Display *display = GDK_DISPLAY_XDISPLAY(gdk_display_get_default());
-    if (_XnrmIsActive && _XnrmQueryScreens && _XnrmIsActive(display)) {
-      screenInfo = _XnrmQueryScreens(display, &numScreens);
-    }
+    // initialize this screen object
+    screen->Init(gdkScreen, i, numScreens);
   }
 
-  // screenInfo == nullptr if either Xinerama couldn't be loaded or
-  // isn't running on the current display
-  if (!screenInfo || numScreens == 1) {
-    numScreens = 1;
-#endif
-    RefPtr<nsScreenGtk> screen;
-
-    if (mCachedScreenArray.Count() > 0) {
-      screen = static_cast<nsScreenGtk*>(mCachedScreenArray[0]);
-    } else {
-      screen = new nsScreenGtk();
-      if (!screen || !mCachedScreenArray.AppendObject(screen)) {
-        return NS_ERROR_OUT_OF_MEMORY;
-      }
-    }
-
-    screen->Init(mRootWindow);
-#ifdef MOZ_X11
-  }
-  // If Xinerama is enabled and there's more than one screen, fill
-  // in the info for all of the screens.  If that's not the case
-  // then nsScreenGTK() defaults to the screen width + height
-  else {
-#ifdef DEBUG
-    printf("Xinerama superpowers activated for %d screens!\n", numScreens);
-#endif
-    for (int i = 0; i < numScreens; ++i) {
-      RefPtr<nsScreenGtk> screen;
-      if (mCachedScreenArray.Count() > i) {
-        screen = static_cast<nsScreenGtk*>(mCachedScreenArray[i]);
-      } else {
-        screen = new nsScreenGtk();
-        if (!screen || !mCachedScreenArray.AppendObject(screen)) {
-          return NS_ERROR_OUT_OF_MEMORY;
-        }
-      }
-
-      // initialize this screen object
-      screen->Init(&screenInfo[i]);
-    }
-  }
   // Remove any screens that are no longer present.
   while (mCachedScreenArray.Count() > numScreens) {
     mCachedScreenArray.RemoveObjectAt(mCachedScreenArray.Count() - 1);
   }
 
-  if (screenInfo) {
-    XFree(screenInfo);
-  }
-#endif
-
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsScreenManagerGtk :: ScreenForId ( uint32_t aId, nsIScreen **outScreen )
 {
   *outScreen = nullptr;
 
@@ -292,20 +231,26 @@ NS_IMETHODIMP
 nsScreenManagerGtk :: GetPrimaryScreen(nsIScreen * *aPrimaryScreen) 
 {
   nsresult rv;
   rv =  EnsureInit();
   if (NS_FAILED(rv)) {
     NS_ERROR("nsScreenManagerGtk::EnsureInit() failed from GetPrimaryScreen");
     return rv;
   }
+#if (MOZ_WIDGET_GTK == 3)
+  // gdk_screen_get_primary_monitor is GTK 2.20+
+  GdkScreen* screen = gdk_screen_get_default();
+  gint index = gdk_screen_get_primary_monitor(screen);
+  *aPrimaryScreen = mCachedScreenArray.SafeObjectAt(index);
+#else
   *aPrimaryScreen = mCachedScreenArray.SafeObjectAt(0);
+#endif
   NS_IF_ADDREF(*aPrimaryScreen);
   return NS_OK;
-  
 } // GetPrimaryScreen
 
 
 //
 // GetNumberOfScreens
 //
 // Returns how many physical screens are available.
 //

--- a/widget/gtk/nsScreenManagerGtk.h
+++ b/widget/gtk/nsScreenManagerGtk.h
@@ -36,17 +36,15 @@ public:
 private:
   virtual ~nsScreenManagerGtk();
 
   nsresult EnsureInit();
 
   // Cached screen array.  Its length is the number of screens we have.
   nsCOMArray<nsIScreen> mCachedScreenArray;
 
-  PRLibrary *mXineramalib;
-
   GdkWindow *mRootWindow;
 #ifdef MOZ_X11
   Atom mNetWorkareaAtom;
 #endif
 };
 
 #endif  // nsScreenManagerGtk_h___