This is a composition of these patches for Firefox 60:

https://bugzilla.mozilla.org/show_bug.cgi?id=1441665
https://bugzilla.mozilla.org/show_bug.cgi?id=1456898
https://bugzilla.mozilla.org/show_bug.cgi?id=1457309
https://bugzilla.mozilla.org/show_bug.cgi?id=1457691

which fix popup window placement at CSD window mode.

diff -up firefox-60.0/widget/gtk/gtk3drawing.cpp.old firefox-60.0/widget/gtk/gtk3drawing.cpp
--- firefox-60.0/widget/gtk/gtk3drawing.cpp.old	2018-04-26 22:07:36.000000000 +0200
+++ firefox-60.0/widget/gtk/gtk3drawing.cpp	2018-04-30 11:59:06.750866104 +0200
@@ -38,6 +38,16 @@ static ToolbarGTKMetrics sToolbarMetrics
 #define GTK_STATE_FLAG_CHECKED (1 << 11)
 #endif
 
+static GtkBorder
+operator+=(GtkBorder& first, const GtkBorder& second)
+{
+    first.left += second.left;
+    first.right += second.right;
+    first.top += second.top;
+    first.bottom += second.bottom;
+    return first;
+}
+
 static gint
 moz_gtk_get_tab_thickness(GtkStyleContext *style);
 
@@ -3056,6 +3066,76 @@ GetScrollbarMetrics(GtkOrientation aOrie
     return metrics;
 }
 
+/*
+ * get_shadow_width() from gtkwindow.c is not public so we need
+ * to implement it.
+ */
+bool
+GetCSDDecorationSize(GtkWindow *aGtkWindow, GtkBorder* aDecorationSize)
+{
+    GtkStyleContext* context = gtk_widget_get_style_context(GTK_WIDGET(aGtkWindow));
+    bool solidDecorations = gtk_style_context_has_class(context, "solid-csd");
+    context = GetStyleContext(solidDecorations ?
+                              MOZ_GTK_WINDOW_DECORATION_SOLID :
+                              MOZ_GTK_WINDOW_DECORATION);
+
+    /* Always sum border + padding */
+    GtkBorder padding;
+    GtkStateFlags state = gtk_style_context_get_state(context);
+    gtk_style_context_get_border(context, state, aDecorationSize);
+    gtk_style_context_get_padding(context, state, &padding);
+    *aDecorationSize += padding;
+
+    // Available on GTK 3.20+.
+    static auto sGtkRenderBackgroundGetClip =
+        (void (*)(GtkStyleContext*, gdouble, gdouble, gdouble, gdouble, GdkRectangle*))
+        dlsym(RTLD_DEFAULT, "gtk_render_background_get_clip");
+
+    GtkBorder margin;
+    gtk_style_context_get_margin(context, state, &margin);
+
+    GtkBorder extents = {0, 0, 0, 0};
+    if (sGtkRenderBackgroundGetClip) {
+        /* Get shadow extents but combine with style margin; use the bigger value.
+         */
+        GdkRectangle clip;
+        sGtkRenderBackgroundGetClip(context, 0, 0, 0, 0, &clip);
+
+        extents.top = -clip.y;
+        extents.right = clip.width + clip.x;
+        extents.bottom = clip.height + clip.y;
+        extents.left = -clip.x;
+
+        // Margin is used for resize grip size - it's not present on
+        // popup windows.
+        if (gtk_window_get_window_type(aGtkWindow) != GTK_WINDOW_POPUP) {
+            extents.top = MAX(extents.top, margin.top);
+            extents.right = MAX(extents.right, margin.right);
+            extents.bottom = MAX(extents.bottom, margin.bottom);
+            extents.left = MAX(extents.left, margin.left);
+        }
+    } else {
+        /* If we can't get shadow extents use decoration-resize-handle instead
+         * as a workaround. This is inspired by update_border_windows()
+         * from gtkwindow.c although this is not 100% accurate as we emulate
+         * the extents here.
+         */
+        gint handle;
+        gtk_widget_style_get(GetWidget(MOZ_GTK_WINDOW),
+                             "decoration-resize-handle", &handle,
+                             NULL);
+
+        extents.top = handle + margin.top;
+        extents.right = handle + margin.right;
+        extents.bottom = handle + margin.bottom;
+        extents.left = handle + margin.left;
+    }
+
+    *aDecorationSize += extents;
+
+    return (sGtkRenderBackgroundGetClip != nullptr);
+}
+
 /* cairo_t *cr argument has to be a system-cairo. */
 gint
 moz_gtk_widget_paint(WidgetNodeType widget, cairo_t *cr,
diff -up firefox-60.0/widget/gtk/gtkdrawing.h.old firefox-60.0/widget/gtk/gtkdrawing.h
--- firefox-60.0/widget/gtk/gtkdrawing.h.old	2018-04-26 22:07:35.000000000 +0200
+++ firefox-60.0/widget/gtk/gtkdrawing.h	2018-04-30 11:59:06.750866104 +0200
@@ -334,6 +334,10 @@ typedef enum {
    */
   MOZ_GTK_HEADER_BAR_BUTTON_MAXIMIZE_RESTORE,
 
+  /* Client-side window decoration node. Available on GTK 3.20+. */
+  MOZ_GTK_WINDOW_DECORATION,
+  MOZ_GTK_WINDOW_DECORATION_SOLID,
+
   MOZ_GTK_WIDGET_NODE_COUNT
 } WidgetNodeType;
 
@@ -606,4 +610,17 @@ GetToolbarButtonMetrics(WidgetNodeType a
 int
 GetGtkHeaderBarButtonLayout(WidgetNodeType* aButtonLayout, int aMaxButtonNums);
 
+/**
+ * Get size of CSD window extents of given GtkWindow.
+ *
+ * aGtkWindow      [IN]  Decorated window.
+ * aDecorationSize [OUT] Returns calculated (or estimated) decoration
+ *                       size of given aGtkWindow.
+ *
+ * returns:    True if we have extract decoration size (for GTK 3.20+)
+ *             False if we have only an estimation (for GTK+ before  3.20+)
+ */
+bool
+GetCSDDecorationSize(GtkWindow *aGtkWindow, GtkBorder* aDecorationSize);
+
 #endif
diff -up firefox-60.0/widget/gtk/nsWindow.cpp.old firefox-60.0/widget/gtk/nsWindow.cpp
--- firefox-60.0/widget/gtk/nsWindow.cpp.old	2018-04-30 12:01:38.788343254 +0200
+++ firefox-60.0/widget/gtk/nsWindow.cpp	2018-04-30 12:00:01.012679502 +0200
@@ -127,6 +127,7 @@ using namespace mozilla::widget;
 #endif
 
 #include "nsShmImage.h"
+#include "gtkdrawing.h"
 
 #include "nsIDOMWheelEvent.h"
 
@@ -3360,6 +3361,10 @@ nsWindow::OnWindowStateEvent(GtkWidget *
           aEvent->new_window_state & GDK_WINDOW_STATE_FULLSCREEN);
       }
     }
+
+    if (mDrawInTitlebar && mCSDSupportLevel == CSD_SUPPORT_CLIENT) {
+        UpdateClientOffsetForCSDWindow();
+    }
 }
 
 void
@@ -6556,6 +6561,32 @@ nsWindow::ClearCachedResources()
     }
 }
 
+/* nsWindow::UpdateClientOffsetForCSDWindow() is designed to be called from
+ * paint code to update mClientOffset any time. It also propagates
+ * the mClientOffset to child tabs.
+ *
+ * It works only for CSD decorated GtkWindow.
+ */
+void
+nsWindow::UpdateClientOffsetForCSDWindow()
+{
+    // _NET_FRAME_EXTENTS is not set on client decorated windows,
+    // so we need to read offset between mContainer and toplevel mShell
+    // window.
+    if (mSizeState == nsSizeMode_Normal) {
+        GtkBorder decorationSize;
+        GetCSDDecorationSize(GTK_WINDOW(mShell), &decorationSize);
+        mClientOffset = nsIntPoint(decorationSize.left, decorationSize.top);
+    } else {
+        mClientOffset = nsIntPoint(0, 0);
+    }
+
+    // Send a WindowMoved notification. This ensures that TabParent
+    // picks up the new client offset and sends it to the child process
+    // if appropriate.
+    NotifyWindowMoved(mBounds.x, mBounds.y);
+}
+
 nsresult
 nsWindow::SetNonClientMargins(LayoutDeviceIntMargin &aMargins)
 {
@@ -6628,6 +6659,13 @@ nsWindow::SetDrawsInTitlebar(bool aState
           mNeedsShow = true;
           NativeResize();
 
+          // When we use system titlebar setup managed by Gtk+ we also get
+          // _NET_FRAME_EXTENTS property for our toplevel window so we can't
+          // update the client offset it here.
+          if (aState) {
+              UpdateClientOffsetForCSDWindow();
+          }
+
           gtk_widget_destroy(tmpWindow);
       }
   }
diff -up firefox-60.0/widget/gtk/nsWindow.h.old firefox-60.0/widget/gtk/nsWindow.h
--- firefox-60.0/widget/gtk/nsWindow.h.old	2018-04-26 22:07:35.000000000 +0200
+++ firefox-60.0/widget/gtk/nsWindow.h	2018-04-30 11:57:33.656146337 +0200
@@ -456,6 +456,8 @@ private:
     nsIWidgetListener* GetListener();
     bool               IsComposited() const;
 
+    void               UpdateClientOffsetForCSDWindow();
+
     GtkWidget          *mShell;
     MozContainer       *mContainer;
     GdkWindow          *mGdkWindow;
diff -up firefox-60.0/widget/gtk/WidgetStyleCache.cpp.old firefox-60.0/widget/gtk/WidgetStyleCache.cpp
--- firefox-60.0/widget/gtk/WidgetStyleCache.cpp.old	2018-04-26 22:07:35.000000000 +0200
+++ firefox-60.0/widget/gtk/WidgetStyleCache.cpp	2018-04-30 13:07:04.170056312 +0200
@@ -1285,6 +1285,22 @@ GetCssNodeStyleInternal(WidgetNodeType a
           "MOZ_GTK_HEADER_BAR_BUTTON_RESTORE is used as an icon only!");
       return nullptr;
     }
+    case MOZ_GTK_WINDOW_DECORATION:
+    {
+      GtkStyleContext* parentStyle =
+          CreateSubStyleWithClass(MOZ_GTK_WINDOW, "csd");
+      style = CreateCSSNode("decoration", parentStyle);
+      g_object_unref(parentStyle);
+      break;
+    }
+    case MOZ_GTK_WINDOW_DECORATION_SOLID:
+    {
+      GtkStyleContext* parentStyle =
+          CreateSubStyleWithClass(MOZ_GTK_WINDOW, "solid-csd");
+      style = CreateCSSNode("decoration", parentStyle);
+      g_object_unref(parentStyle);
+      break;
+    }
     default:
       return GetWidgetRootStyle(aNodeType);
   }