View > Toolbars > Customize or right-click either the Menu bar or the Nav bar and select Customize

edmeister -- Exactly what I needed. I just kept looking under Tools>. Didn't think of View>. Thanks so much. -- cindysdad

You're welcome. Please mark this thread as Solved.

Oops, I spoke to soon. That indeed makes the close button visible. However, when I click on it, it only opens or closes "Add-in-One Sidebar". It does not effect the tabs.

Perhaps I should repost as a separate question?

Did you check the settings of that extension in Tools > Add-ons > Extensions?