Uncheck this. see screenshot

Note that some extensions support containers and in that case you may not be able to disable containers. In this case you should see a notifications above this setting: An extension requires Container Tabs

I confirmed the message. I am using the Firefox add-on "Tree Style Tab", but if I don't disable this add-on, will container tabs also be disabled?

As long as an extension that uses containers is enabled then you can't disable containers. If you disable the extension(s) then you can disable or enable containers in Setteings and choose whether you see container related items in menus.

Thank you for your advice. If I can't hide the container tab without disabling the add-on, I'll continue to use it as is. Thank you for your advice.