Tabs are on the left side in vertical order not on top in horizontal order.

Hello, Do you have a '''settings''' icon (⚙) on the left side at the bottom? If so, make sure Vertical tabs in the settings is unckeked. If a little vertical bar is remaining have a look at Mozilla documentation. There is a pictogram to the left of the arrows near the address bar. It's here to hide the little bar. More infos at : https://support.mozilla.org/en-US/kb/use-sidebar-access-tools-and-vertical-tabs Also it could be an extension (add-on).