--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -691,21 +691,16 @@ nsPresContext::AppUnitsPerDevPixelChange
 
 void
 nsPresContext::PreferenceChanged(const char* aPrefName)
 {
   nsDependentCString prefName(aPrefName);
   if (prefName.EqualsLiteral("layout.css.dpi") ||
       prefName.EqualsLiteral("layout.css.devPixelsPerPx")) {
 
-    // We can't use a separate observer, callback, or var cache
-    // Because they don't guarantee the order of function calls.
-    // We have to update the scale override value first.
-    nsIWidget::ScaleOverrideChanged();
-
     int32_t oldAppUnitsPerDevPixel = AppUnitsPerDevPixel();
     if (mDeviceContext->CheckDPIChange() && mShell) {
       nsCOMPtr<nsIPresShell> shell = mShell;
       // Re-fetch the view manager's window dimensions in case there's a deferred
       // resize which hasn't affected our mVisibleArea yet
       nscoord oldWidthAppUnits, oldHeightAppUnits;
       RefPtr<nsViewManager> vm = shell->GetViewManager();
       if (!vm) {

--- a/widget/nsBaseWidget.cpp
+++ b/widget/nsBaseWidget.cpp
@@ -568,86 +568,32 @@ CSSToLayoutDeviceScale nsIWidget::GetDef
 
   if (devPixelsPerCSSPixel <= 0.0) {
     devPixelsPerCSSPixel = GetDefaultScaleInternal();
   }
 
   return CSSToLayoutDeviceScale(devPixelsPerCSSPixel);
 }
 
-// The number of device pixels per CSS pixel. A value <= 0 means choose
-// automatically based on the DPI. A positive value is used as-is. This effectively
-// controls the size of a CSS "px".
-static double sDevPixelsPerCSSPixel = -1.0;
-
-/* static */
-void nsIWidget::ScaleOverrideChanged()
-{
-  nsAdoptingCString prefString = Preferences::GetCString("layout.css.devPixelsPerPx");
-  if (!prefString.IsEmpty()) {
-    sDevPixelsPerCSSPixel = PR_strtod(prefString, nullptr);
-  } else {
-    sDevPixelsPerCSSPixel = -1.0;
-  }
-}
-
-class ProfileChangeObserver final : public nsIObserver
-{
-public:
-  NS_DECL_ISUPPORTS
-  NS_DECL_NSIOBSERVER
-  static void Init();
-private:
-  ProfileChangeObserver();
-  ~ProfileChangeObserver();
-};
-
-ProfileChangeObserver::ProfileChangeObserver()
-{
-}
-
-ProfileChangeObserver::~ProfileChangeObserver()
-{
-}
-
-/* static */
-void ProfileChangeObserver::Init()
-{
-  nsCOMPtr<nsIObserver> profileChangeObserver = new ProfileChangeObserver();
-  nsCOMPtr<nsIObserverService> obs = services::GetObserverService();
-  obs->AddObserver(profileChangeObserver, "profile-do-change", false);
-  // We don't have to observe XPCOM shutdown to remove the observer
-  // explicitly. Also, we don't have to hold a strong reference to the
-  // observer on our own. The observer service will do them for us.
-}
-
-NS_IMPL_ISUPPORTS(ProfileChangeObserver,
-                  nsIObserver)
-
-NS_IMETHODIMP
-ProfileChangeObserver::Observe(nsISupports* aSubject, const char* aTopic,
-                               const char16_t* aData)
-{
-  if (!strcmp(aTopic, "profile-do-change")) {
-    nsIWidget::ScaleOverrideChanged();
-  }
-  return NS_OK;
-}
-
 /* static */
 double nsIWidget::DefaultScaleOverride()
 {
+  // The number of device pixels per CSS pixel. A value <= 0 means choose
+  // automatically based on the DPI. A positive value is used as-is. This effectively
+  // controls the size of a CSS "px".
+  static float devPixelsPerCSSPixel = -1.0f;
+
   static bool valueCached = false;
   if (!valueCached) {
-    ScaleOverrideChanged();
-    ProfileChangeObserver::Init();
+    Preferences::AddFloatVarCache(&devPixelsPerCSSPixel,
+                                  "layout.css.devPixelsPerPx", -1.0f);
     valueCached = true;
   }
 
-  return sDevPixelsPerCSSPixel;
+  return devPixelsPerCSSPixel;
 }
 
 //-------------------------------------------------------------------------
 //
 // Add a child to the list of children
 //
 //-------------------------------------------------------------------------
 void nsBaseWidget::AddChild(nsIWidget* aChild)

--- a/widget/nsIWidget.h
+++ b/widget/nsIWidget.h
@@ -568,22 +568,16 @@ class nsIWidget : public nsISupports
      * default number of device pixels per CSS pixel to use. This should
      * depend on OS/platform settings such as the Mac's "UI scale factor"
      * or Windows' "font DPI". This will take into account Gecko preferences
      * overriding the system setting.
      */
     mozilla::CSSToLayoutDeviceScale GetDefaultScale();
 
     /**
-     * Cache the scale override value. Call whenever the scale override
-     * value is changed.
-     */
-    static void ScaleOverrideChanged();
-
-    /**
      * Return the Gecko override of the system default scale, if any;
      * returns <= 0.0 if the system scale should be used as-is.
      * nsIWidget::GetDefaultScale() [above] takes this into account.
      * It is exposed here so that code that wants to check for a
      * default-scale override without having a widget on hand can
      * easily access the same value.
      * Note that any scale override is a browser-wide value, whereas
      * the default GetDefaultScale value (when no override is present)