Index: mozilla/browser/base/moz.build
===================================================================
--- mozilla.orig/browser/base/moz.build
+++ mozilla/browser/base/moz.build
@@ -60,7 +60,7 @@ DEFINES['APP_LICENSE_BLOCK'] = '%s/conte
 if CONFIG['MOZ_WIDGET_TOOLKIT'] in ('windows', 'gtk3', 'cocoa'):
     DEFINES['CONTEXT_COPY_IMAGE_CONTENTS'] = 1
 
-if CONFIG['MOZ_WIDGET_TOOLKIT'] in ('windows', 'cocoa'):
+if CONFIG['MOZ_WIDGET_TOOLKIT'] in ('windows', 'cocoa', 'gtk3'):
     DEFINES['CAN_DRAW_IN_TITLEBAR'] = 1
 
 if CONFIG['MOZ_WIDGET_TOOLKIT'] in ('windows', 'gtk3'):
Index: mozilla/toolkit/modules/moz.build
===================================================================
--- mozilla.orig/toolkit/modules/moz.build
+++ mozilla/toolkit/modules/moz.build
@@ -265,7 +265,7 @@ EXTRA_JS_MODULES.sessionstore += [
     'sessionstore/Utils.jsm',
 ]
 
-if CONFIG['MOZ_WIDGET_TOOLKIT'] in ('windows', 'cocoa'):
+if CONFIG['MOZ_WIDGET_TOOLKIT'] in ('windows', 'cocoa', 'gtk3'):
     DEFINES['CAN_DRAW_IN_TITLEBAR'] = 1
 
 if CONFIG['MOZ_WIDGET_TOOLKIT'] in ('windows', 'gtk3'):
Index: mozilla/widget/gtk/nsWindow.cpp
===================================================================
--- mozilla.orig/widget/gtk/nsWindow.cpp
+++ mozilla/widget/gtk/nsWindow.cpp
@@ -6857,9 +6857,62 @@ nsWindow::GetCSDSupportLevel() {
     if (sCSDSupportLevel != CSD_SUPPORT_UNKNOWN) {
         return sCSDSupportLevel;
     }
-            
-    // Disabled due to Bug 1440461
-    sCSDSupportLevel = CSD_SUPPORT_NONE;
+
+    const char* currentDesktop = getenv("XDG_CURRENT_DESKTOP");
+    if (currentDesktop) {
+        if (strstr(currentDesktop, "GNOME") != nullptr) {
+            sCSDSupportLevel = CSD_SUPPORT_FULL;
+        } else if (strstr(currentDesktop, "XFCE") != nullptr) {
+            sCSDSupportLevel = CSD_SUPPORT_FLAT;
+        } else if (strstr(currentDesktop, "X-Cinnamon") != nullptr) {
+            sCSDSupportLevel = CSD_SUPPORT_FULL;
+        } else if (strstr(currentDesktop, "KDE") != nullptr) {
+            sCSDSupportLevel = CSD_SUPPORT_FLAT;
+        } else if (strstr(currentDesktop, "LXDE") != nullptr) {
+            sCSDSupportLevel = CSD_SUPPORT_FLAT;
+        } else if (strstr(currentDesktop, "openbox") != nullptr) {
+            sCSDSupportLevel = CSD_SUPPORT_FLAT;
+        } else if (strstr(currentDesktop, "i3") != nullptr) {
+            sCSDSupportLevel = CSD_SUPPORT_NONE;
+        } else if (strstr(currentDesktop, "MATE") != nullptr) {
+            sCSDSupportLevel = CSD_SUPPORT_FLAT;
+        } else if (strstr(currentDesktop, "Unity") != nullptr) {
+            sCSDSupportLevel = CSD_SUPPORT_FLAT;
+        } else if (strstr(currentDesktop, "Pantheon") != nullptr) {
+            sCSDSupportLevel = CSD_SUPPORT_FULL;
+        } else if (strstr(currentDesktop, "LXQt") != nullptr) {
+            sCSDSupportLevel = CSD_SUPPORT_FULL;
+        } else {
+// Release or beta builds are not supposed to be broken
+// so disable titlebar rendering on untested/unknown systems.
+#if defined(RELEASE_OR_BETA)
+            sCSDSupportLevel = CSD_SUPPORT_NONE;
+#else
+            sCSDSupportLevel = CSD_SUPPORT_FLAT;
+#endif
+        }
+    } else {
+        sCSDSupportLevel = CSD_SUPPORT_NONE;
+    }
+
+    // We don't support CSD_SUPPORT_FULL on Wayland
+    if (!GDK_IS_X11_DISPLAY(gdk_display_get_default()) &&
+        sCSDSupportLevel == CSD_SUPPORT_FULL) {
+        sCSDSupportLevel = CSD_SUPPORT_FLAT;
+    }
+
+    // Allow MOZ_GTK_TITLEBAR_DECORATION to override our heuristics
+    const char* decorationOverride = getenv("MOZ_GTK_TITLEBAR_DECORATION");
+    if (decorationOverride) {
+        if (strcmp(decorationOverride, "none") == 0) {
+            sCSDSupportLevel = CSD_SUPPORT_NONE;
+        } else if (strcmp(decorationOverride, "client") == 0) {
+            sCSDSupportLevel = CSD_SUPPORT_FLAT;
+        } else if (strcmp(decorationOverride, "system") == 0) {
+            sCSDSupportLevel = CSD_SUPPORT_FULL;
+        }
+    }
+
     return sCSDSupportLevel;
 }