--- a/mail/base/content/mainNavigationToolbox.inc
+++ b/mail/base/content/mainNavigationToolbox.inc
@@ -942,14 +942,12 @@
 #endif
 
       <!-- Help -->
 #include helpMenu.inc
 
       <spacer id="menubar_spacer" flex="100%"/>
       </menubar>
     </toolbaritem>
-#ifdef CAN_DRAW_IN_TITLEBAR
 #ifndef XP_MACOSX
     <hbox class="titlebar-placeholder" type="caption-buttons" ordinal="1000"/>
 #endif
-#endif
   </toolbar>

--- a/mail/base/content/messenger.xul
+++ b/mail/base/content/messenger.xul
@@ -308,33 +308,31 @@
     <menuitem label="&openAddressBookDock.label;" id="tasksOpenAddressBook"
               oncommand="openAddressBookDock();"/>
     <menuitem label="&dockOptions.label;" id="tasksMenuDockOptions"
               oncommand="openDockOptions();"/>
   </menupopup>
 </popupset>
 #endif
 
-#ifdef CAN_DRAW_IN_TITLEBAR
 <vbox id="titlebar">
   <hbox id="titlebar-content">
     <spacer id="titlebar-spacer" flex="1"/>
     <hbox id="titlebar-buttonbox-container" align="start">
       <hbox id="titlebar-buttonbox">
         <toolbarbutton class="titlebar-button" id="titlebar-min" oncommand="window.minimize();"/>
         <toolbarbutton class="titlebar-button" id="titlebar-max" oncommand="onTitlebarMaxClick();"/>
         <toolbarbutton class="titlebar-button" id="titlebar-close" oncommand="window.close()"/>
       </hbox>
     </hbox>
 #ifdef XP_MACOSX
     <hbox id="titlebar-fullscreen-button"/>
 #endif
   </hbox>
 </vbox>
-#endif
 
   <!-- navigation-toolbox with main menubar and tabs toolbar -->
 #include mainNavigationToolbox.inc
 
     <toolbar id="tabs-toolbar">
       <!-- class tabmail-tabs is unused and only maintained for add-ons. -->
       <tabs flex="1"
             id="tabmail-tabs"
@@ -367,26 +365,25 @@
                      id="alltabs-button"
                      type="menu"
                      hidden="true"
                      tooltiptext="&listAllTabs.label;">
         <menupopup class="tabs-alltabs-popup"
                    id="alltabs-popup" position="after_end"
                    tabcontainer="tabmail-tabs"/>
       </toolbarbutton>
-#ifdef CAN_DRAW_IN_TITLEBAR
+
       <hbox class="titlebar-placeholder" type="caption-buttons"
 #ifndef XP_MACOSX
             ordinal="1000"
 #endif
             />
 #ifdef XP_MACOSX
       <hbox class="titlebar-placeholder" type="fullscreen-button"/>
 #endif
-#endif
     </toolbar>
 
   </toolbox>
 
   <!-- XXX This extension point (tabmail-container) is only temporary!
        Horizontal space shouldn't be wasted if it isn't absolutely critical.
        A mechanism for adding sidebar panes will be added in bug 476154. -->
   <hbox id="tabmail-container" flex="1">

--- a/mail/base/content/msgMail3PaneWindow.js
+++ b/mail/base/content/msgMail3PaneWindow.js
@@ -1646,83 +1646,87 @@ function InitPageMenu(menuPopup, event) 
   PageMenuParent.buildAndAddToPopup(menuPopup.triggerNode, menuPopup);
 
   if (menuPopup.children.length == 0)
     event.preventDefault();
 }
 
 var TabsInTitlebar = {
   init() {
-    if (AppConstants.CAN_DRAW_IN_TITLEBAR) {
-      // Don't trust the initial value of the sizemode attribute; wait for the
-      // resize event.
-      this._readPref();
-      Services.prefs.addObserver(this._drawInTitlePref, this);
-      Services.prefs.addObserver(this._autoHidePref, this);
+    // Don't trust the initial value of the sizemode attribute; wait for the
+    // resize event.
+    this._readPref();
+    Services.prefs.addObserver(this._drawInTitlePref, this);
+    Services.prefs.addObserver(this._autoHidePref, this);
 
-      this.allowedBy("sizemode", false);
-      window.addEventListener("resize", function (event) {
-        if (event.target != window)
-          return;
-        TabsInTitlebar.allowedBy("sizemode", true);
-      });
-
-      // Always disable on unsupported GTK versions.
-      if (AppConstants.MOZ_WIDGET_TOOLKIT == "gtk3") {
-        this.allowedBy("gtk", window.matchMedia("(-moz-gtk-csd-available)"));
-      }
+    this.allowedBy("sizemode", false);
+    window.addEventListener("resize", function (event) {
+      if (event.target != window)
+        return;
+      TabsInTitlebar.allowedBy("sizemode", true);
+    });
 
-      // We need to update the appearance of the titlebar when the menu changes
-      // from the active to the inactive state. We can't, however, rely on
-      // DOMMenuBarInactive, because the menu fires this event and then removes
-      // the inactive attribute after an event-loop spin.
-      //
-      // Because updating the appearance involves sampling the heights and
-      // margins of various elements, it's important that the layout be more or
-      // less settled before updating the titlebar. So instead of listening to
-      // DOMMenuBarActive and DOMMenuBarInactive, we use a MutationObserver to
-      // watch the "invalid" attribute directly.
-      let menu = document.getElementById("mail-toolbar-menubar2");
-      this._menuObserver = new MutationObserver(this._onMenuMutate);
-      this._menuObserver.observe(menu, {attributes: true});
+    // We need to update the appearance of the titlebar when the menu changes
+    // from the active to the inactive state. We can't, however, rely on
+    // DOMMenuBarInactive, because the menu fires this event and then removes
+    // the inactive attribute after an event-loop spin.
+    //
+    // Because updating the appearance involves sampling the heights and
+    // margins of various elements, it's important that the layout be more or
+    // less settled before updating the titlebar. So instead of listening to
+    // DOMMenuBarActive and DOMMenuBarInactive, we use a MutationObserver to
+    // watch the "invalid" attribute directly.
+    let menu = document.getElementById("mail-toolbar-menubar2");
+    this._menuObserver = new MutationObserver(this._onMenuMutate);
+    this._menuObserver.observe(menu, {attributes: true});
 
-      let sizeMode = document.getElementById("messengerWindow");
-      this._sizeModeObserver = new MutationObserver(this._onSizeModeMutate);
-      this._sizeModeObserver.observe(sizeMode, {attributes: true});
+    let sizeMode = document.getElementById("messengerWindow");
+    this._sizeModeObserver = new MutationObserver(this._onSizeModeMutate);
+    this._sizeModeObserver.observe(sizeMode, {attributes: true});
 
-      this._initialized = true;
-      if (this._updateOnInit) {
-        // We don't need to call this with 'true', even if original calls
-        // (before init()) did, because this will be the first call and so
-        // we will update anyway.
-        this._update();
-      }
+    this._initialized = true;
+    if (this._updateOnInit) {
+      // We don't need to call this with 'true', even if original calls
+      // (before init()) did, because this will be the first call and so
+      // we will update anyway.
+      this._update();
     }
   },
 
   allowedBy(condition, allow) {
-    if (AppConstants.CAN_DRAW_IN_TITLEBAR) {
-      if (allow) {
-        if (condition in this._disallowed) {
-          delete this._disallowed[condition];
-          this._update(true);
-        }
-      } else {
-        if (!(condition in this._disallowed)) {
-          this._disallowed[condition] = null;
-          this._update(true);
-        }
+    if (allow) {
+      if (condition in this._disallowed) {
+        delete this._disallowed[condition];
+        this._update(true);
+      }
+    } else {
+      if (!(condition in this._disallowed)) {
+        this._disallowed[condition] = null;
+        this._update(true);
       }
     }
   },
 
   updateAppearance: function updateAppearance(aForce) {
-    if (AppConstants.CAN_DRAW_IN_TITLEBAR) {
-      this._update(aForce);
+    this._update(aForce);
+  },
+
+  get systemSupported() {
+    let isSupported = false;
+    switch (AppConstants.MOZ_WIDGET_TOOLKIT) {
+      case "windows":
+      case "cocoa":
+        isSupported = true;
+        break;
+      case "gtk3":
+        isSupported = window.matchMedia("(-moz-gtk-csd-available)");
+        break;
     }
+    delete this.systemSupported;
+    return this.systemSupported = isSupported;
   },
 
   get enabled() {
     return document.documentElement.getAttribute("tabsintitlebar") == "true";
   },
 
   observe(subject, topic, data) {
     if (topic == "nsPref:changed")
@@ -1792,17 +1796,18 @@ var TabsInTitlebar = {
       // still changing in the consequent "fullscreen" event. Code there will
       // call this function again when everything is ready.
       // See browser-fullScreen.js: FullScreen.toggle and bug 1173768.
       if (oldSizeMode == "fullscreen") {
         return;
       }
     }
 
-    let allowed = (Object.keys(this._disallowed)).length == 0;
+    let allowed = this.systemSupported &&
+                  (Object.keys(this._disallowed)).length == 0;
 
     let titlebar = $("titlebar");
     let titlebarContent = $("titlebar-content");
     let titlebarButtons = $("titlebar-buttonbox");
     let menubar = $("mail-toolbar-menubar2");
 
     if (allowed) {
       // We set the tabsintitlebar attribute first so that our CSS for
@@ -1925,41 +1930,37 @@ var TabsInTitlebar = {
   },
 
   _sizePlaceholder(type, width) {
     Array.forEach(document.querySelectorAll(".titlebar-placeholder[type='" + type + "']"),
                   function(node) { node.style.width = width + "px"; });
   },
 
   uninit() {
-    if (AppConstants.CAN_DRAW_IN_TITLEBAR) {
-      this._initialized = false;
-      Services.prefs.removeObserver(this._drawInTitlePref, this);
-      Services.prefs.removeObserver(this._autoHidePref, this);
-      this._menuObserver.disconnect();
-    }
+    this._initialized = false;
+    Services.prefs.removeObserver(this._drawInTitlePref, this);
+    Services.prefs.removeObserver(this._autoHidePref, this);
+    this._menuObserver.disconnect();
   }
 };
 
-if (AppConstants.CAN_DRAW_IN_TITLEBAR) {
-  function updateTitlebarDisplay() {
-    if (AppConstants.platform == "macosx") {
-      if (TabsInTitlebar.enabled) {
-        document.documentElement.setAttribute("chromemargin", "0,-1,-1,-1");
-        document.documentElement.removeAttribute("drawtitle");
-      } else {
-        document.documentElement.removeAttribute("chromemargin");
-        document.documentElement.setAttribute("drawtitle", "true");
-      }
-    } else if (TabsInTitlebar.enabled) {
-      // not OS X
-      document.documentElement.setAttribute("chromemargin", "0,2,2,2");
+function updateTitlebarDisplay() {
+  if (AppConstants.platform == "macosx") {
+    if (TabsInTitlebar.enabled) {
+      document.documentElement.setAttribute("chromemargin", "0,-1,-1,-1");
+      document.documentElement.removeAttribute("drawtitle");
     } else {
       document.documentElement.removeAttribute("chromemargin");
+      document.documentElement.setAttribute("drawtitle", "true");
     }
+  } else if (TabsInTitlebar.enabled) {
+    // not OS X
+    document.documentElement.setAttribute("chromemargin", "0,2,2,2");
+  } else {
+    document.documentElement.removeAttribute("chromemargin");
   }
 }
 
 /* Draw */
 function onTitlebarMaxClick() {
   if (window.windowState == window.STATE_MAXIMIZED)
     window.restore();
   else

--- a/mail/base/moz.build
+++ b/mail/base/moz.build
@@ -16,15 +16,12 @@ DEFINES['mozreltopsrcdir'] = CONFIG['moz
 DEFINES['APP_LICENSE_BLOCK'] = '%s/content/overrides/app-license.html' % SRCDIR
 
 if CONFIG['MOZILLA_OFFICIAL']:
     DEFINES['OFFICIAL_BUILD'] = 1
 
 if CONFIG['MOZ_WIDGET_TOOLKIT'] in ('windows', 'gtk3', 'cocoa'):
     DEFINES['HAVE_SHELL_SERVICE'] = 1
 
-if CONFIG['MOZ_WIDGET_TOOLKIT'] in ('windows', 'cocoa', 'gtk3'):
-    DEFINES['CAN_DRAW_IN_TITLEBAR'] = 1
-
 if CONFIG['MOZ_UPDATER']:
     DEFINES['MOZ_UPDATER'] = 1
 
 DEFINES['TOOLKIT_DIR'] = "%s/toolkit" % (CONFIG['moztopsrcdir'],)