Edit: One user proposed to set widget.macos.native-context-menus to false in about:config, but the issue I’m trying to solve is the `` context menus not looking native, which I’m trying to fix with CSS. So I don’t want to set it to false. I filed a bug report since the menus are supposed to be native when set to true.

This is the page to see the select elment, you change the CSS using the browser toolbox (not the dev tools). I tried setting border-radius on both the option elements and the context menu itself, but nothing has worked.