--- a/mobile/android/components/SessionStore.js
+++ b/mobile/android/components/SessionStore.js
@@ -699,22 +699,49 @@ SessionStore.prototype = {
     // Save the current document resolution.
     let zoom = { value: 1 };
     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);
+
     // Save zoom and scroll data.
     data.scrolldata = scrolldata;
     log("onTabScroll() ran for tab " + aWindow.BrowserApp.getTabForBrowser(aBrowser).id);
     this.saveStateDelayed();
   },
 
+  _getViewportInfo: function ss_getViewportInfo(aDisplayWidth, aDisplayHeight, aWindow) {
+    let viewportInfo = {};
+    let defaultZoom = {}, allowZoom = {}, minZoom = {}, maxZoom ={},
+        width = {}, height = {}, autoSize = {};
+    aWindow.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(
+      Ci.nsIDOMWindowUtils).getViewportInfo(aDisplayWidth, aDisplayHeight,
+        defaultZoom, allowZoom, minZoom, maxZoom, width, height, autoSize);
+
+    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;
+
+    return viewportInfo;
+  },
+
   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
       let delay = Math.max(minimalDelay, 2000);
       if (delay > 0) {
@@ -1309,25 +1336,46 @@ SessionStore.prototype = {
   },
 
   /**
   * 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) {
-      log("_restoreZoom(), resolution: " + aScrollData.zoom.resolution);
+      let recalculatedZoom = this._recalculateZoom(aScrollData.zoom);
+      log("_restoreZoom(), resolution: " + recalculatedZoom);
+
       let utils = aBrowser.contentWindow.QueryInterface(
         Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowUtils);
       // Restore zoom level.
-      utils.setRestoreResolution(aScrollData.zoom.resolution);
+      utils.setRestoreResolution(recalculatedZoom);
     }
   },
 
   /**
+  * 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);
     }
   },