Problem with FireFox Quantum.
It is not possible to display two panels, left and right at the same time.
In the previous version firefox (56.*) it was possible to display bookmarks in the left panel and on the right opened tabs at the same time.
Now, in FireFox Quantum, only one panel is available, left or right at one time. Open tabs are displayed at the top and do not depend on whether the Tree tab is enabled or not.