Firefox : override website's JavaScript to a local machine JavaScript

Question

There is a userContent.css available in Firefox to customize / override a website's CSS to a new one, but what about the JavaScript?

How can I override a particular website's JavaScript or some part of the JavaScript of a website to a local machine's JavaScript in Firefox?

Any greasemonkey tips or something else native tips without any addons? or are there any plugins available for the same?

Answer 1

It may be possible to override the original JavaScript functionality of a website in some cases. I know of examples using bookmarklets to turn off copy-pasting restrictions on web form fields and replacing dynamic blinking alerts imposed by a website in tab with custom text of your choice.