For those with vision impairments the options you need to set to true in about:config are:

widget.gtk.global-menu.enabled

widget.gtk.global-menu.wayland.enabled

Presumably you would only need the second option if you are using Wayland. I tried it (am using Plasma on Fedora 42) and it worked but I’m not interested in it so I disabled it after I made sure all the menus gave a drop down. YMMV.