diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index 64a118d75811..8b71aa6dff85 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -352,7 +352,6 @@ nsDocShell::nsDocShell(BrowsingContext* aBrowsingContext,
       mAllowAuth(mItemType == typeContent),
       mAllowKeywordFixup(false),
       mDisableMetaRefreshWhenInactive(false),
-      mDeviceSizeIsPageSize(false),
       mWindowDraggingAllowed(false),
       mInFrameSwap(false),
       mFiredUnloadEvent(false),
@@ -442,9 +441,6 @@ bool nsDocShell::Initialize() {
       Preferences::GetBool("browser.meta_refresh_when_inactive.disabled",
                            mDisableMetaRefreshWhenInactive);
 
-  mDeviceSizeIsPageSize = Preferences::GetBool(
-      "docshell.device_size_is_page_size", mDeviceSizeIsPageSize);
-
   if (nsCOMPtr<nsIObserverService> serv = services::GetObserverService()) {
     const char* msg = mItemType == typeContent ? NS_WEBNAVIGATION_CREATE
                                                : NS_CHROME_WEBNAVIGATION_CREATE;
@@ -3196,26 +3192,6 @@ nsDocShell::GetIsNavigating(bool* aOut) {
   return NS_OK;
 }
 
-NS_IMETHODIMP
-nsDocShell::SetDeviceSizeIsPageSize(bool aValue) {
-  if (mDeviceSizeIsPageSize != aValue) {
-    mDeviceSizeIsPageSize = aValue;
-    RefPtr<nsPresContext> presContext = GetPresContext();
-    if (presContext) {
-      presContext->MediaFeatureValuesChanged(
-          {MediaFeatureChangeReason::DeviceSizeIsPageSizeChange},
-          MediaFeatureChangePropagation::JustThisDocument);
-    }
-  }
-  return NS_OK;
-}
-
-NS_IMETHODIMP
-nsDocShell::GetDeviceSizeIsPageSize(bool* aValue) {
-  *aValue = mDeviceSizeIsPageSize;
-  return NS_OK;
-}
-
 void nsDocShell::ClearFrameHistory(nsISHEntry* aEntry) {
   MOZ_ASSERT(!mozilla::SessionHistoryInParent());
   RefPtr<ChildSHistory> rootSH = GetRootSessionHistory();
diff --git a/docshell/base/nsDocShell.h b/docshell/base/nsDocShell.h
index 8694ae41501b..7eb46fa817ee 100644
--- a/docshell/base/nsDocShell.h
+++ b/docshell/base/nsDocShell.h
@@ -1316,7 +1316,6 @@ class nsDocShell final : public nsDocLoader,
   bool mAllowKeywordFixup : 1;
   bool mDisableMetaRefreshWhenInactive : 1;
   bool mIsAppTab : 1;
-  bool mDeviceSizeIsPageSize : 1;
   bool mWindowDraggingAllowed : 1;
   bool mInFrameSwap : 1;
 
diff --git a/docshell/base/nsIDocShell.idl b/docshell/base/nsIDocShell.idl
index a291a2e8d232..68f32e968c7e 100644
--- a/docshell/base/nsIDocShell.idl
+++ b/docshell/base/nsIDocShell.idl
@@ -622,18 +622,6 @@ interface nsIDocShell : nsIDocShellTreeItem
 
   [noscript,notxpcom,nostdcall] Document getExtantDocument();
 
-  /**
-   * 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 Mode and B2G Simulator.
-   *
-   * Default is False.
-   * Default value can be overriden with
-   * docshell.device_size_is_page_size pref.
-   */
-  [infallible] attribute boolean deviceSizeIsPageSize;
-
   /**
    * Notify DocShell when the browser is about to start executing JS, and after
    * that execution has stopped.  This only occurs when the Timeline devtool
diff --git a/layout/base/nsPresContext.cpp b/layout/base/nsPresContext.cpp
index 5ca34d81134e..93746258875d 100644
--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -2815,11 +2815,6 @@ nscoord nsPresContext::PhysicalMillimetersToAppUnits(float aMM) const {
       inches * float(DeviceContext()->AppUnitsPerPhysicalInch()));
 }
 
-bool nsPresContext::IsDeviceSizePageSize() {
-  nsIDocShell* docShell = GetDocShell();
-  return docShell && docShell->GetDeviceSizeIsPageSize();
-}
-
 uint64_t nsPresContext::GetRestyleGeneration() const {
   if (!mRestyleManager) {
     return 0;
diff --git a/layout/base/nsPresContext.h b/layout/base/nsPresContext.h
index d20267147268..066c3c809e2c 100644
--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -1055,8 +1055,6 @@ class nsPresContext : public nsISupports, public mozilla::SupportsWeakPtr {
   bool HasEverBuiltInvisibleText() const { return mHasEverBuiltInvisibleText; }
   void SetBuiltInvisibleText() { mHasEverBuiltInvisibleText = true; }
 
-  bool IsDeviceSizePageSize();
-
   bool HasWarnedAboutTooLargeDashedOrDottedRadius() const {
     return mHasWarnedAboutTooLargeDashedOrDottedRadius;
   }
diff --git a/layout/style/MediaFeatureChange.h b/layout/style/MediaFeatureChange.h
index 70f81f7d1cbd..a766406219cb 100644
--- a/layout/style/MediaFeatureChange.h
+++ b/layout/style/MediaFeatureChange.h
@@ -16,7 +16,7 @@
 
 namespace mozilla {
 
-enum class MediaFeatureChangeReason : uint16_t {
+enum class MediaFeatureChangeReason : uint8_t {
   // The viewport size the document has used has changed.
   //
   // This affects size media queries like min-width.
@@ -34,16 +34,13 @@ enum class MediaFeatureChangeReason : uint16_t {
   // A system metric or multiple have changed. This affects all the media
   // features that expose the presence of a system metric directly.
   SystemMetricsChange = 1 << 5,
-  // The fact of whether the device size is the page size has changed, thus
-  // resolution media queries can change.
-  DeviceSizeIsPageSizeChange = 1 << 6,
   // display-mode changed on the document, thus the display-mode media queries
   // may have changed.
-  DisplayModeChange = 1 << 7,
+  DisplayModeChange = 1 << 6,
   // A preference that affects media query results has changed. For
   // example, changes to document_color_use will affect
   // prefers-contrast.
-  PreferenceChange = 1 << 8,
+  PreferenceChange = 1 << 7,
 };
 
 MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(MediaFeatureChangeReason)
diff --git a/layout/style/ServoStyleSet.h b/layout/style/ServoStyleSet.h
index 8ff9d815c801..394da3ccf65d 100644
--- a/layout/style/ServoStyleSet.h
+++ b/layout/style/ServoStyleSet.h
@@ -29,7 +29,7 @@
 #include "nsSize.h"
 
 namespace mozilla {
-enum class MediaFeatureChangeReason : uint16_t;
+enum class MediaFeatureChangeReason : uint8_t;
 enum class StylePageSizeOrientation : uint8_t;
 enum class StyleRuleChangeKind : uint32_t;