Context menus are native on macOS

Firefox sets widget.macos.native-context-menus to true by default, which means macOS draws right-click menus itself and CSS cannot style them at all. menupopup and menuitem rules have no effect there. They do apply on Windows and Linux.

fxcss doctor reports the setting for your platform, and fxcss watch --native-menus=false switches Firefox to XUL menus so you can work on that styling from a Mac.