Copy/paste about:preferences into the Address Bar then Enter. Under the 1st page which is General go down to Tabs and untick the boxes.

Please let us know if this solved your issue or if need further assistance.

Thank you Pkshadow for the reply.

They were already unmarked. And the tabs are still at the top.

If I close all the tabs, then Firefox Quantum closes.

I am trying to remove tabs completely.