diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index 6f6fa7066cf..d0901337aaa 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -6225,7 +6225,7 @@ nsGlobalWindow::GetMainWidget()
 }
 
 nsIWidget*
-nsGlobalWindow::GetNearestWidget()
+nsGlobalWindow::GetNearestWidget() const
 {
   nsIDocShell* docShell = GetDocShell();
   NS_ENSURE_TRUE(docShell, nullptr);
@@ -6666,8 +6666,16 @@ nsGlobalWindow::FullScreen() const
   nsCOMPtr<nsIDocShellTreeItem> rootItem;
   mDocShell->GetRootTreeItem(getter_AddRefs(rootItem));
   if (rootItem == mDocShell) {
-    // We are the root window. Return our internal value.
-    return mFullScreen;
+    if (!XRE_IsContentProcess()) {
+      // We are the root window. Return our internal value.
+      return mFullScreen;
+    }
+    if (nsCOMPtr<nsIWidget> widget = GetNearestWidget()) {
+      // We are in content process, figure out the value from
+      // the sizemode of the puppet widget.
+      return widget->SizeMode() == nsSizeMode_Fullscreen;
+    }
+    return false;
   }
 
   nsCOMPtr<nsIDOMWindow> window = rootItem->GetWindow();
diff --git a/dom/base/nsGlobalWindow.h b/dom/base/nsGlobalWindow.h
index 164517b9db7..42d52f7ed71 100644
--- a/dom/base/nsGlobalWindow.h
+++ b/dom/base/nsGlobalWindow.h
@@ -1509,7 +1509,7 @@ public:
   bool WindowExists(const nsAString& aName, bool aLookForCallerOnJSStack);
 
   already_AddRefed<nsIWidget> GetMainWidget();
-  nsIWidget* GetNearestWidget();
+  nsIWidget* GetNearestWidget() const;
 
   void Freeze()
   {
diff --git a/dom/base/nsPIDOMWindow.h b/dom/base/nsPIDOMWindow.h
index a67bac99b2a..c6e9fabe753 100644
--- a/dom/base/nsPIDOMWindow.h
+++ b/dom/base/nsPIDOMWindow.h
@@ -368,7 +368,7 @@ public:
   /**
    * Get the docshell in this window.
    */
-  nsIDocShell *GetDocShell()
+  nsIDocShell *GetDocShell() const
   {
     if (mOuterWindow) {
       return mOuterWindow->mDocShell;