Systems using on-demand scrollbar feature (as in your case) do not account for content at the edge of a window and thus will obscure the content when the scrollbar appears. Unfortunately addons do not have a way of detecting system settings.

You still have the option to select the tab(s) and close them from the context menu.

UPDATE: I have added an option in version 1.1.9 where users can statically force a margin to the right of the close buttons. This experimental feature is not an ideal solution, but at least users with Mac on-demand scrollbar setting can access the control buttons.