CSS property `user-select` is used to disable or alter selectability of rendered HTML elements. This userscript unsets all such style properties on a page. Install using the following link: https://gist.github.com/Erquint/5ed7be3f1c62c606d7c514deec4d091d/raw/user-select_unsetter.user.js

user-select_unsetter.user.js

// ==UserScript==
// @name user-select unsetter
// @description Waits for page to fully load once and selectively unsets the `user-select` property of all elements.
// @version 1.0.0
// @namespace gness.na@gmail.com
// @author https://gist.github.com/Erquint
// @homepageURL https://gist.github.com/Erquint/fbea345ae863fea3735fc35f25c753f7
// @updateURL https://gist.githubusercontent.com/Erquint/fbea345ae863fea3735fc35f25c753f7/raw/user-select_unsetter.user.js
// @downloadURL https://gist.githubusercontent.com/Erquint/fbea345ae863fea3735fc35f25c753f7/raw/user-select_unsetter.user.js
// @match *://*/*
// @run-at document-idle
// ==/UserScript==

properties = [
  'user-select',
  '-moz-user-select',
  '-webkit-user-select',
  '-ms-user-select'
]

styles = [...window.document.body.querySelectorAll('*')].map(element => element.style);
[...window.document.styleSheets].forEach(sheet => {
  try{
    [...sheet.cssRules].forEach(rule => {
      styles.push(rule.style);
    });
  }catch(e){};
});
styles = styles.filter(e => e);

styles.forEach(style => {
  properties.forEach(property => {
    if(style.getPropertyValue(property)){style.removeProperty(property)};
  });
});