Remove the context menu in Firefox using userChrome.css.
It aims to provide a context menu removal feature as an alternative to "Menu Wizard" extension in Firefox Quantum (57+).
You can show/hide the menu by add/remov the "/*" at the beginning of the line. You can change it to your liking.
- macOS版 Firefox のコンテキストメニューを編集する - バグ取りの日々
- Native context menu support for Firefox for macOS