diff --git a/browser/devtools/responsivedesign/responsivedesign.jsm b/browser/devtools/responsivedesign/responsivedesign.jsm
index 4a54f003a327..cceda150dd9a 100644
--- a/browser/devtools/responsivedesign/responsivedesign.jsm
+++ b/browser/devtools/responsivedesign/responsivedesign.jsm
@@ -185,6 +185,7 @@ function ResponsiveUI(aWindow, aTab)
                       .getInterface(Ci.nsIWebNavigation)
                       .QueryInterface(Ci.nsIDocShell);
 
+  this._deviceSizeWasPageSize = this.docShell.deviceSizeIsPageSize;
   this.docShell.deviceSizeIsPageSize = true;
 
   try {
@@ -255,7 +256,7 @@ ResponsiveUI.prototype = {
       return;
     this.closing = true;
 
-    this.docShell.deviceSizeIsPageSize = false;
+    this.docShell.deviceSizeIsPageSize = this._deviceSizeWasPageSize;
 
     this.browser.removeEventListener("load", this.bound_onPageLoad, true);
     this.browser.removeEventListener("unload", this.bound_onPageUnload, true);
diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index ad83c81075d6..05b5bbed5f84 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -4951,6 +4951,10 @@ nsDocShell::Create()
         gAddedPreferencesVarCache = true;
     }
 
+    mDeviceSizeIsPageSize =
+        Preferences::GetBool("docshell.device_size_is_page_size",
+                             mDeviceSizeIsPageSize);
+
     nsCOMPtr<nsIObserverService> serv = services::GetObserverService();
     if (serv) {
         const char* msg = mItemType == typeContent ?
diff --git a/docshell/base/nsIDocShell.idl b/docshell/base/nsIDocShell.idl
index eac85028b032..c260f65cd6ec 100644
--- a/docshell/base/nsIDocShell.idl
+++ b/docshell/base/nsIDocShell.idl
@@ -912,7 +912,11 @@ interface nsIDocShell : nsIDocShellTreeItem
    * If deviceSizeIsPageSize is set to true, device-width/height media queries
    * will be calculated from the page size, not the device size.
    *
-   * Used by the Responsive Design View.
+   * Used by the Responsive Design View and B2G Simulator.
+   *
+   * Default is False.
+   * Default value can be overriden with
+   * docshell.device_size_is_page_size pref.
    */
   [infallible] attribute boolean deviceSizeIsPageSize;
 };