--- a/widget/cocoa/nsChildView.mm
+++ b/widget/cocoa/nsChildView.mm
@@ -31,16 +31,17 @@
 #include "nsIInterfaceRequestor.h"
 #include "nsIFile.h"
 #include "nsILocalFileMac.h"
 #include "nsGfxCIID.h"
 #include "nsIDOMSimpleGestureEvent.h"
 #include "nsThemeConstants.h"
 #include "nsIWidgetListener.h"
 #include "nsIPresShell.h"
+#include "nsIScreen.h"
 
 #include "nsDragService.h"
 #include "nsClipboard.h"
 #include "nsCursorManager.h"
 #include "nsWindowMap.h"
 #include "nsCocoaFeatures.h"
 #include "nsCocoaUtils.h"
 #include "nsMenuUtilsX.h"
@@ -818,22 +819,22 @@ nsIWidget*
 nsChildView::GetParent()
 {
   return mParentWidget;
 }
 
 float
 nsChildView::GetDPI()
 {
-  NSWindow* window = [mView window];
-  if (window && [window isKindOfClass:[BaseWindow class]]) {
-    return [(BaseWindow*)window getDPI];
-  }
-
-  return 96.0;
+  float dpi = 96.0;
+  nsCOMPtr<nsIScreen> screen = GetWidgetScreen();
+  if (screen) {
+    screen->GetDpi(&dpi);
+  }
+  return dpi;
 }
 
 void
 nsChildView::Enable(bool aState)
 {
 }
 
 bool nsChildView::IsEnabled() const

--- a/widget/cocoa/nsCocoaWindow.h
+++ b/widget/cocoa/nsCocoaWindow.h
@@ -39,23 +39,16 @@ typedef struct _nsCocoaWindowList {
   NSMutableDictionary* mState;
   BOOL mDrawsIntoWindowFrame;
   NSColor* mActiveTitlebarColor;
   NSColor* mInactiveTitlebarColor;
 
   // Invalidation disabling
   BOOL mDisabledNeedsDisplay;
 
-  // DPI cache. Getting the physical screen size (CGDisplayScreenSize)
-  // is ridiculously slow, so we cache it in the toplevel window for all
-  // descendants to use.
-  //
-  // XXX the dpi cache won't change when moving across different monitors.
-  float mDPI;
-
   NSTrackingArea* mTrackingArea;
 
   NSRect mDirtyRect;
 
   BOOL mBeingShown;
   BOOL mDrawTitle;
   BOOL mBrightTitlebarForeground;
   BOOL mUseMenuStyle;
@@ -63,17 +56,16 @@ typedef struct _nsCocoaWindowList {
 
 - (void)importState:(NSDictionary*)aState;
 - (NSMutableDictionary*)exportState;
 - (void)setDrawsContentsIntoWindowFrame:(BOOL)aState;
 - (BOOL)drawsContentsIntoWindowFrame;
 - (void)setTitlebarColor:(NSColor*)aColor forActiveWindow:(BOOL)aActive;
 - (NSColor*)titlebarColorForActiveWindow:(BOOL)aActive;
 
-- (float)getDPI;
 
 - (void)mouseEntered:(NSEvent*)aEvent;
 - (void)mouseExited:(NSEvent*)aEvent;
 - (void)mouseMoved:(NSEvent*)aEvent;
 - (void)updateTrackingArea;
 - (NSView*)trackingAreaView;
 
 - (void)setBeingShown:(BOOL)aValue;

--- a/widget/cocoa/nsCocoaWindow.mm
+++ b/widget/cocoa/nsCocoaWindow.mm
@@ -2882,41 +2882,16 @@ nsCocoaWindow::GetEditCommands(NativeKey
       listener->WindowDeactivated();
     }
     mToplevelActiveState = false;
   }
 }
 
 @end
 
-static float
-GetDPI(NSWindow* aWindow)
-{
-  NSScreen* screen = [aWindow screen];
-  if (!screen)
-    return 96.0f;
-
-  CGDirectDisplayID displayID =
-    [[[screen deviceDescription] objectForKey:@"NSScreenNumber"] intValue];
-  CGFloat heightMM = ::CGDisplayScreenSize(displayID).height;
-  size_t heightPx = ::CGDisplayPixelsHigh(displayID);
-  if (heightMM < 1 || heightPx < 1) {
-    // Something extremely bogus is going on
-    return 96.0f;
-  }
-
-  float dpi = heightPx / (heightMM / MM_PER_INCH_FLOAT);
-
-  // Account for HiDPI mode where Cocoa's "points" do not correspond to real
-  // device pixels
-  CGFloat backingScale = GetBackingScaleFactor(aWindow);
-
-  return dpi * backingScale;
-}
-
 @interface NSView(FrameViewMethodSwizzling)
 - (NSPoint)FrameView__closeButtonOrigin;
 - (NSPoint)FrameView__fullScreenButtonOrigin;
 @end
 
 @implementation NSView(FrameViewMethodSwizzling)
 
 - (NSPoint)FrameView__closeButtonOrigin
@@ -3060,17 +3035,16 @@ static NSMutableSet *gSwizzledFrameViewC
 - (id)initWithContentRect:(NSRect)aContentRect styleMask:(NSUInteger)aStyle backing:(NSBackingStoreType)aBufferingType defer:(BOOL)aFlag
 {
   mDrawsIntoWindowFrame = NO;
   [super initWithContentRect:aContentRect styleMask:aStyle backing:aBufferingType defer:aFlag];
   mState = nil;
   mActiveTitlebarColor = nil;
   mInactiveTitlebarColor = nil;
   mDisabledNeedsDisplay = NO;
-  mDPI = GetDPI(self);
   mTrackingArea = nil;
   mDirtyRect = NSZeroRect;
   mBeingShown = NO;
   mDrawTitle = NO;
   mBrightTitlebarForeground = NO;
   mUseMenuStyle = NO;
   [self updateTrackingArea];
 
@@ -3208,21 +3182,16 @@ static const NSString* kStateCollectionB
   }
 }
 
 - (NSColor*)titlebarColorForActiveWindow:(BOOL)aActive
 {
   return aActive ? mActiveTitlebarColor : mInactiveTitlebarColor;
 }
 
-- (float)getDPI
-{
-  return mDPI;
-}
-
 - (NSView*)trackingAreaView
 {
   NSView* contentView = [self contentView];
   return [contentView superview] ? [contentView superview] : contentView;
 }
 
 - (ChildView*)mainChildView
 {