--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -551,24 +551,27 @@ nsDOMWindowUtils::SetResolutionAndScaleT
   }
 
   presShell->SetResolutionAndScaleTo(aResolution);
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
-nsDOMWindowUtils::SetRestoreResolution(float aResolution)
+nsDOMWindowUtils::SetRestoreResolution(float aResolution,
+                                       uint32_t aDisplayWidth,
+                                       uint32_t aDisplayHeight)
 {
   nsIPresShell* presShell = GetPresShell();
   if (!presShell) {
     return NS_ERROR_FAILURE;
   }
 
-  presShell->SetRestoreResolution(aResolution);
+  presShell->SetRestoreResolution(aResolution,
+    LayoutDeviceIntSize(aDisplayWidth, aDisplayHeight));
 
   return NS_OK;
 }
 
 NS_IMETHODIMP
 nsDOMWindowUtils::GetResolution(float* aResolution)
 {
   nsIPresShell* presShell = GetPresShell();

--- a/dom/interfaces/base/nsIDOMWindowUtils.idl
+++ b/dom/interfaces/base/nsIDOMWindowUtils.idl
@@ -224,20 +224,25 @@ interface nsIDOMWindowUtils : nsISupport
    * for pinch-zoom on mobile platforms.
    *
    * The caller of this method must have chrome privileges.
    */
   void setResolutionAndScaleTo(in float aResolution);
 
   /**
    * Set a resolution on the presShell which is the "restored" from history.
+   * The display dimensions are compared to their current values and used
+   * to scale the resolution value if necessary, e.g. if the device was
+   * rotated between saving and restoring of the session data.
    * This resolution should be used when painting for the first time. Calling
    * this too late may have no effect.
    */
-  void setRestoreResolution(in float aResolution);
+  void setRestoreResolution(in float aResolution,
+                            in uint32_t aDisplayWidth,
+                            in uint32_t aDisplayHeight);
 
   /**
    * Whether the resolution has been set by the user.
    * This gives a way to check whether the provided resolution is the default
    * value or restored from a previous session.
    *
    * Can only be accessed with chrome privileges.
    */

--- a/layout/base/MobileViewportManager.cpp
+++ b/layout/base/MobileViewportManager.cpp
@@ -80,19 +80,23 @@ MobileViewportManager::Destroy()
     observerService->RemoveObserver(this, BEFORE_FIRST_PAINT.Data());
   }
 
   mDocument = nullptr;
   mPresShell = nullptr;
 }
 
 void
-MobileViewportManager::SetRestoreResolution(float aResolution)
+MobileViewportManager::SetRestoreResolution(float aResolution,
+                                            LayoutDeviceIntSize aDisplaySize)
 {
   mRestoreResolution = Some(aResolution);
+  ScreenIntSize restoreDisplaySize = ViewAs<ScreenPixel>(aDisplaySize,
+    PixelCastJustification::LayoutDeviceIsScreenForBounds);
+  mRestoreDisplaySize = Some(restoreDisplaySize);
 }
 
 void
 MobileViewportManager::RequestReflow()
 {
   MVM_LOG("%p: got a reflow request\n", this);
   RefreshViewportSize(false);
 }
@@ -190,17 +194,29 @@ MobileViewportManager::UpdateResolution(
 {
   CSSToLayoutDeviceScale cssToDev =
       mPresShell->GetPresContext()->CSSToDevPixelScale();
   LayoutDeviceToLayerScale res(mPresShell->GetResolution());
 
   if (mIsFirstPaint) {
     CSSToScreenScale defaultZoom;
     if (mRestoreResolution) {
-      defaultZoom = CSSToScreenScale(mRestoreResolution.value() * cssToDev.scale);
+    LayoutDeviceToLayerScale restoreResolution(mRestoreResolution.value());
+      if (mRestoreDisplaySize) {
+        CSSSize prevViewport = mDocument->GetViewportInfo(mRestoreDisplaySize.value()).GetSize();
+        float restoreDisplayWidthChangeRatio = (mRestoreDisplaySize.value().width > 0)
+          ? (float)aDisplaySize.width / (float)mRestoreDisplaySize.value().width : 1.0f;
+
+        restoreResolution =
+          ScaleResolutionWithDisplayWidth(restoreResolution,
+                                          restoreDisplayWidthChangeRatio,
+                                          aViewport,
+                                          prevViewport);
+      }
+      defaultZoom = CSSToScreenScale(restoreResolution.scale * cssToDev.scale);
       MVM_LOG("%p: restored zoom is %f\n", this, defaultZoom.scale);
       defaultZoom = ClampZoom(defaultZoom, aViewportInfo);
     } else {
       defaultZoom = aViewportInfo.GetDefaultZoom();
       MVM_LOG("%p: default zoom from viewport is %f\n", this, defaultZoom.scale);
       if (!aViewportInfo.IsDefaultZoomValid()) {
         defaultZoom = MaxScaleRatio(ScreenSize(aDisplaySize), aViewport);
         MVM_LOG("%p: Intrinsic computed zoom is %f\n", this, defaultZoom.scale);

--- a/layout/base/MobileViewportManager.h
+++ b/layout/base/MobileViewportManager.h
@@ -24,18 +24,22 @@ public:
   NS_DECL_NSIOBSERVER
 
   MobileViewportManager(nsIPresShell* aPresShell,
                         nsIDocument* aDocument);
   void Destroy();
 
   /* Provide a resolution to use during the first paint instead of the default
    * resolution computed from the viewport info metadata. This is in the same
-   * "units" as the argument to nsDOMWindowUtils::SetResolutionAndScaleTo. */
-  void SetRestoreResolution(float aResolution);
+   * "units" as the argument to nsDOMWindowUtils::SetResolutionAndScaleTo.
+   * Also takes the previous display dimensions as they were at the time the
+   * resolution was stored in order to correctly adjust the resolution if the
+   * device was rotated in the meantime. */
+  void SetRestoreResolution(float aResolution,
+                            mozilla::LayoutDeviceIntSize aDisplaySize);
 
   /* Notify the MobileViewportManager that a reflow was requested in the
    * presShell.*/
   void RequestReflow();
 
   /* Notify the MobileViewportManager that the resolution on the presShell was
    * updated, and the SPCSPS needs to be updated. */
   void ResolutionUpdated();
@@ -81,12 +85,13 @@ private:
   nsCOMPtr<nsIDocument> mDocument;
   nsIPresShell* MOZ_NON_OWNING_REF mPresShell; // raw ref since the presShell owns this
   nsCOMPtr<nsIDOMEventTarget> mEventTarget;
   bool mIsFirstPaint;
   bool mPainted;
   mozilla::LayoutDeviceIntSize mDisplaySize;
   mozilla::CSSSize mMobileViewportSize;
   mozilla::Maybe<float> mRestoreResolution;
+  mozilla::Maybe<mozilla::ScreenIntSize> mRestoreDisplaySize;
 };
 
 #endif
 

--- a/layout/base/nsIPresShell.h
+++ b/layout/base/nsIPresShell.h
@@ -43,16 +43,17 @@
 #include "nsWeakReference.h"
 #include <stdio.h> // for FILE definition
 #include "nsChangeHint.h"
 #include "nsRefPtrHashtable.h"
 #include "nsClassHashtable.h"
 #include "nsPresArena.h"
 #include "nsMargin.h"
 #include "nsFrameState.h"
+#include "Units.h"
 #include "Visibility.h"
 
 #ifdef MOZ_B2G
 #include "nsIHardwareKeyHandler.h"
 #endif
 
 class nsDocShell;
 class nsIDocument;
@@ -1441,17 +1442,18 @@ public:
    * SetResolutionAndScaleTo(), and set to false by a call to SetResolution().
    */
   virtual bool ScaleToResolution() const = 0;
 
   /**
    * Used by session restore code to restore a resolution before the first
    * paint.
    */
-  virtual void SetRestoreResolution(float aResolution) = 0;
+  virtual void SetRestoreResolution(float aResolution,
+                                    mozilla::LayoutDeviceIntSize aDisplaySize) = 0;
 
   /**
    * Returns whether we are in a DrawWindow() call that used the
    * DRAWWINDOW_DO_NOT_FLUSH flag.
    */
   bool InDrawWindowNotFlushing() const
   { return mRenderFlags & STATE_DRAWWINDOW_NOT_FLUSHING; }
 

--- a/layout/base/nsPresShell.cpp
+++ b/layout/base/nsPresShell.cpp
@@ -5628,20 +5628,21 @@ float PresShell::GetCumulativeNonRootSca
       currentShell = parentCtx->PresShell();
     } else {
       currentShell = nullptr;
     }
   }
   return resolution;
 }
 
-void PresShell::SetRestoreResolution(float aResolution)
+void PresShell::SetRestoreResolution(float aResolution,
+                                     LayoutDeviceIntSize aDisplaySize)
 {
   if (mMobileViewportManager) {
-    mMobileViewportManager->SetRestoreResolution(aResolution);
+    mMobileViewportManager->SetRestoreResolution(aResolution, aDisplaySize);
   }
 }
 
 void PresShell::SetRenderingState(const RenderingState& aState)
 {
   if (mRenderFlags != aState.mRenderFlags) {
     // Rendering state changed in a way that forces us to flush any
     // retained layers we might already have.

--- a/layout/base/nsPresShell.h
+++ b/layout/base/nsPresShell.h
@@ -235,17 +235,18 @@ public:
     return SetResolutionImpl(aResolution, /* aScaleToResolution = */ false);
   }
   virtual nsresult SetResolutionAndScaleTo(float aResolution) override {
     return SetResolutionImpl(aResolution, /* aScaleToResolution = */ true);
   }
   virtual bool ScaleToResolution() const override;
   virtual float GetCumulativeResolution() override;
   virtual float GetCumulativeNonRootScaleResolution() override;
-  virtual void SetRestoreResolution(float aResolution) override;
+  virtual void SetRestoreResolution(float aResolution,
+                                    mozilla::LayoutDeviceIntSize aDisplaySize) override;
 
   //nsIViewObserver interface
 
   virtual void Paint(nsView* aViewToPaint, const nsRegion& aDirtyRegion,
                      uint32_t aFlags) override;
   virtual nsresult HandleEvent(nsIFrame* aFrame,
                                mozilla::WidgetGUIEvent* aEvent,
                                bool aDontRetargetEvents,

--- a/mobile/android/components/SessionStore.js
+++ b/mobile/android/components/SessionStore.js
@@ -739,47 +739,37 @@ SessionStore.prototype = {
     content.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(
       Ci.nsIDOMWindowUtils).getResolution(zoom);
     scrolldata.zoom = {};
     scrolldata.zoom.resolution = zoom.value;
     log("onTabScroll() zoom level: " + zoom.value);
 
     // Save some data that'll help in adjusting the zoom level
     // when restoring in a different screen orientation.
-    let viewportInfo = this._getViewportInfo(aWindow.outerWidth, aWindow.outerHeight, content);
-    scrolldata.zoom.autoSize = viewportInfo.autoSize;
-    log("onTabScroll() autoSize: " + scrolldata.zoom.autoSize);
-    scrolldata.zoom.windowWidth = aWindow.outerWidth;
-    log("onTabScroll() windowWidth: " + scrolldata.zoom.windowWidth);
+    scrolldata.zoom.displaySize = this._getContentViewerSize(content);
+    log("onTabScroll() displayWidth: " + scrolldata.zoom.displaySize.width);
 
     // Save zoom and scroll data.
     data.scrolldata = scrolldata;
     log("onTabScroll() ran for tab " + aWindow.BrowserApp.getTabForBrowser(aBrowser).id);
     let evt = new Event("SSTabScrollCaptured", {"bubbles":true, "cancelable":false});
     aBrowser.dispatchEvent(evt);
     this.saveStateDelayed();
   },
 
-  _getViewportInfo: function ss_getViewportInfo(aDisplayWidth, aDisplayHeight, aWindow) {
-    let viewportInfo = {};
-    let defaultZoom = {}, allowZoom = {}, minZoom = {}, maxZoom ={},
-        width = {}, height = {}, autoSize = {};
+  _getContentViewerSize: function ss_getContentViewerSize(aWindow) {
+    let displaySize = {};
+    let width = {}, height = {};
     aWindow.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(
-      Ci.nsIDOMWindowUtils).getViewportInfo(aDisplayWidth, aDisplayHeight,
-        defaultZoom, allowZoom, minZoom, maxZoom, width, height, autoSize);
+      Ci.nsIDOMWindowUtils).getContentViewerSize(width, height);
 
-    viewportInfo.defaultZoom = defaultZoom.value;
-    viewportInfo.allowZoom = allowZoom.value;
-    viewportInfo.minZoom = maxZoom.value;
-    viewportInfo.maxZoom = maxZoom.value;
-    viewportInfo.width = width.value;
-    viewportInfo.height = height.value;
-    viewportInfo.autoSize = autoSize.value;
+    displaySize.width = width.value;
+    displaySize.height = height.value;
 
-    return viewportInfo;
+    return displaySize;
   },
 
   saveStateDelayed: function ss_saveStateDelayed() {
     if (!this._saveTimer) {
       // Interval until the next disk operation is allowed
       let minimalDelay = this._lastSaveTime + this._interval - Date.now();
 
       // If we have to wait, set a timer, otherwise saveState directly
@@ -1371,51 +1361,34 @@ SessionStore.prototype = {
   _restoreTextData: function ss_restoreTextData(aFormData, aBrowser) {
     if (aFormData) {
       log("_restoreTextData()");
       FormData.restoreTree(aBrowser.contentWindow, aFormData);
     }
   },
 
   /**
-  * Restores the zoom level of the window. This needs to be called before
-  * first paint/load (whichever comes first) to take any effect.
-  */
+   * Restores the zoom level of the window. This needs to be called before
+   * first paint/load (whichever comes first) to take any effect.
+   */
   _restoreZoom: function ss_restoreZoom(aScrollData, aBrowser) {
-    if (aScrollData && aScrollData.zoom) {
-      let recalculatedZoom = this._recalculateZoom(aScrollData.zoom);
-      log("_restoreZoom(), resolution: " + recalculatedZoom);
+    if (aScrollData && aScrollData.zoom && aScrollData.zoom.displaySize) {
+      log("_restoreZoom(), resolution: " + aScrollData.zoom.resolution +
+          ", old displayWidth: " + aScrollData.zoom.displaySize.width);
 
       let utils = aBrowser.contentWindow.QueryInterface(
         Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowUtils);
       // Restore zoom level.
-      utils.setRestoreResolution(recalculatedZoom);
+      utils.setRestoreResolution(aScrollData.zoom.resolution,
+                                 aScrollData.zoom.displaySize.width,
+                                 aScrollData.zoom.displaySize.height);
     }
   },
 
   /**
-  * Recalculates the zoom level to account for a changed display width,
-  * e.g. because the device was rotated.
-  */
-  _recalculateZoom: function ss_recalculateZoom(aZoomData) {
-    let browserWin = Services.wm.getMostRecentWindow("navigator:browser");
-
-    // Pages with "width=device-width" won't need any zoom level scaling.
-    if (!aZoomData.autoSize) {
-      let oldWidth = aZoomData.windowWidth;
-      let newWidth = browserWin.outerWidth;
-      if (oldWidth != newWidth && oldWidth > 0 && newWidth > 0) {
-        log("_recalculateZoom(), old resolution: " + aZoomData.resolution);
-        return newWidth / oldWidth * aZoomData.resolution;
-      }
-    }
-    return aZoomData.resolution;
-  },
-
-  /**
   * Takes serialized scroll positions and restores them into the given browser.
   */
   _restoreScrollPosition: function ss_restoreScrollPosition(aScrollData, aBrowser) {
     if (aScrollData) {
       log("_restoreScrollPosition()");
       ScrollPosition.restoreTree(aBrowser.contentWindow, aScrollData);
     }
   },