@nulll thanks for reporting the issue. When I use and edit Quick Docs at desktop, navigation menu is displayed on the left pane. My laptop has a 13 inch screen and runs latest Firefox that comes with KDE Plasma.
You’re right - my Samsung Z Fold 3 doesn’t show navigation menu.
I didn’t realize how it would be rendered in my phone.
As usual with mobile devices, the menu is located on the far left of the top bar (below the blue header bar). It is not a familiar “burger”, but a stylized bulleted list.