On GNOME Firefox runs with disabled system titlebar by default. It saves horizontal space on wide screens but also removes control over window, traditionally provided by Window manager and desktop environment.
GNOME allows to set titlebar actions by gnome-tweaks tool, you can define window actions for double click by first mouse button, middle click and secondary button. These choices are not followed by Firefox if system titlebar is off because Firefox integrates titlebar with browser tab strip and performs build-in tasks like open/close new tab or toggle maximize.
However Firefox 124 improves it and follows mouse button double click action defined by GNOME so you change it as you wish.
You also can define titlebar action for middle mouse button click, which opens a new tab by default. Set widget.gtk.titlebar-action-middle-click-enabled at about:config and it should work then.
Martin Stransky's Blog 