diff --git a/browser/base/content/browser-init.js b/browser/base/content/browser-init.js
index cc62392ab0..0c68a0e991 100644
--- a/browser/base/content/browser-init.js
+++ b/browser/base/content/browser-init.js
@@ -1568,7 +1568,7 @@ var gBrowserInit = {
     updateBookmarkToolbarVisibility();
 
     // Set a sane starting width/height for all resolutions on new profiles.
-    if (ChromeUtils.shouldResistFingerprinting("RoundWindowSize", null)) {
+    if (true || ChromeUtils.shouldResistFingerprinting("RoundWindowSize", null)) {
       // When the fingerprinting resistance is enabled, making sure that we don't
       // have a maximum window to interfere with generating rounded window dimensions.
       document.documentElement.setAttribute("sizemode", "normal");
@@ -1796,6 +1796,47 @@ var gBrowserInit = {
       )?.removeAttribute("key");
     }
 
+    // Set default size
+    window.resizeTo(1280, 1040);
+
+    // Hijack the outer window size
+    let outerWidth, outerHeight;
+    if ((outerWidth = ChromeUtils.camouGetConfig("window.outerWidth"))) {
+      document.documentElement.style.setProperty('width', outerWidth + 'px');
+      browser.style.setProperty('width', outerWidth + 'px');
+      window.resizeTo(outerWidth, window.outerHeight);
+    }
+    if ((outerHeight = ChromeUtils.camouGetConfig("window.outerHeight"))) {
+      document.documentElement.style.setProperty('height', outerHeight + 'px');
+      browser.style.setProperty('height', outerHeight + 'px');
+      window.resizeTo(window.outerWidth, outerHeight);
+    }
+    browser.style.setProperty('box-sizing', 'content-box');
+
+    // Hijack the inner window size
+    let innerWidth = ChromeUtils.camouGetConfig("window.innerWidth") || ChromeUtils.camouGetConfig("document.body.clientWidth");
+    let innerHeight = ChromeUtils.camouGetConfig("window.innerHeight") || ChromeUtils.camouGetConfig("document.body.clientHeight");
+
+    if (innerWidth || innerHeight) {
+      let win_inner_style = document.createElement('style');
+      win_inner_style.innerHTML = `
+        .browserStack {
+          ${innerWidth ? `width: ${innerWidth}px !important;` : ''}
+          ${innerHeight ? `height: ${innerHeight}px !important;` : ''}
+          overflow: auto;
+          contain: size;
+          scrollbar-width: none;
+        }
+      `;
+      document.head.appendChild(win_inner_style);
+    }
+
+    if (innerWidth && innerHeight && !(outerWidth || outerHeight)) {
+      let stackRect = __browserStack.getBoundingClientRect();
+      let toolbarTop = stackRect.y;
+      window.resizeBy(width - innerWidth, height + toolbarTop - innerHeight);
+    }
+
     this._loadHandled = true;
   },
 
diff --git a/dom/base/ChromeUtils.cpp b/dom/base/ChromeUtils.cpp
index 0df1cd3c9b..49844441b0 100644
--- a/dom/base/ChromeUtils.cpp
+++ b/dom/base/ChromeUtils.cpp
@@ -5,6 +5,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "ChromeUtils.h"
+#include "MaskConfig.hpp"
 
 #include "JSOracleParent.h"
 #include "js/CallAndConstruct.h"  // JS::Call
@@ -2082,6 +2083,16 @@ void ChromeUtils::GetAllPossibleUtilityActorNames(GlobalObject& aGlobal,
   }
 }
 
+/* static */
+int32_t ChromeUtils::CamouGetConfig(GlobalObject& aGlobal,
+                                    const nsAString& aVarName) {
+  NS_ConvertUTF16toUTF8 utf8VarName(aVarName);
+  if (auto value = MaskConfig::GetInt32(utf8VarName.get())) {
+    return value.value();
+  }
+  return 0;
+}
+
 /* static */
 bool ChromeUtils::ShouldResistFingerprinting(
     GlobalObject& aGlobal, JSRFPTarget aTarget,
diff --git a/dom/base/ChromeUtils.h b/dom/base/ChromeUtils.h
index 4c2d043ecb..1d717b656e 100644
--- a/dom/base/ChromeUtils.h
+++ b/dom/base/ChromeUtils.h
@@ -305,6 +305,8 @@ class ChromeUtils {
   static void GetAllPossibleUtilityActorNames(GlobalObject& aGlobal,
                                               nsTArray<nsCString>& aNames);
 
+  static int32_t CamouGetConfig(GlobalObject& aGlobal, const nsAString& aVarName);
+
   static bool ShouldResistFingerprinting(
       GlobalObject& aGlobal, JSRFPTarget aTarget,
       const Nullable<uint64_t>& aOverriddenFingerprintingSettings);
diff --git a/dom/chrome-webidl/ChromeUtils.webidl b/dom/chrome-webidl/ChromeUtils.webidl
index 3ccb125a1e..02b231d1c8 100644
--- a/dom/chrome-webidl/ChromeUtils.webidl
+++ b/dom/chrome-webidl/ChromeUtils.webidl
@@ -747,6 +747,11 @@ partial namespace ChromeUtils {
   [ChromeOnly]
   readonly attribute unsigned long aliveUtilityProcesses;
 
+  /**
+   * Get the value from Camoufox MaskConfig.
+   */
+  long camouGetConfig(DOMString varName);
+
   /**
    * Get a list of all possible Utility process Actor Names ; mostly useful to
    * perform testing and ensure about:processes display is sound and misses no