Firefox 82: tab bar merged into window title bar

If you go into Customize, you can enable the title bar again. It's a checkbox in the lower left corner on the Customize dialogue.

Firefox uses legacy window decoration, i.e., the classical title bar, which is provided by the window manager. The title bar can be shown or hidden through the Firefox preferences dialog. If you hide it, you loose that right-click window context menu, unfortunately.

To directly have the context menu for windows available, your only option is to enable the title bar. Alternatively, you can still show it using the shortcut key Alt+Space. A shortcut key to directly move a window down is Super+Shift+Arrows. However, this will not send the window away, but take you along with the window to the next workspace.