--- a/browser/base/content/browser-tabsintitlebar.js
+++ b/browser/base/content/browser-tabsintitlebar.js
@@ -163,17 +163,17 @@ var TabsInTitlebar = {
 
       // Try to avoid reflows in this code by calculating dimensions first and
       // then later set the properties affecting layout together in a batch.
 
       // Get the height of the tabs toolbar:
       let fullTabsHeight = rect($("TabsToolbar")).height;
 
       // Buttons first:
-      let captionButtonsBoxWidth = rect($("titlebar-buttonbox-container")).width;
+      let captionButtonsBoxWidth = rect($("titlebar-buttonbox")).width;
 
       let secondaryButtonsWidth, menuHeight, fullMenuHeight, menuStyles;
       if (AppConstants.platform == "macosx") {
         secondaryButtonsWidth = rect($("titlebar-secondary-buttonbox")).width;
         // No need to look up the menubar stuff on OS X:
         menuHeight = 0;
         fullMenuHeight = 0;
       } else {