You can use Menu Wizard to hide contextual menu items. https://addons.mozilla.org/en-US/firefox/addon/s3menu-wizard/

Thank you for your response. I have tried the addon to hide the menu. But if the user tries to disable the addon and reload the page it would still be available. Is there any other way?

Hack the code in Firefox program files. And then repeat every time Firefox gets an update and the omni.ja file is replaced.

I am not exactly sure what you mean by hacking the code in firefox? I will install the add-on you have told hoping the users didn't find out that add-on is installed. Thank you so much response the-edmeister