As has already been stated, setting user-select: none in the page's CSS is what disables selection of text on a page. The easiest way to remove that text selection block would be through a user script like the following that overrides the setting:

// ==UserScript==
// @name         Force Select
// @version      1.0
// @description  Stop sites from disabling selection of text
// @author       You
// @match        *://*/*
// @grant        none
// ==/UserScript==

(function() {
  'use strict';

  let style = document.createElement('style');
  style.innerHTML = '*{ user-select: auto !important; }';

  document.body.appendChild(style);
})();

Note: This will apply to every page if left enabled, which might not be desirable in all situations.

As has already been stated, setting user-select: none in the page's CSS is what disables selection of text on a page. The easiest way to remove that text selection block would be through a user script like the following that overrides the setting:

// ==UserScript==
// @name         Force Select
// @version      1.0
// @description  Stop sites from disabling selection of text
// @author       You
// @match        *://*/*
// @grant        none
// ==/UserScript==

(function() {
  'use strict';

  let style = document.createElement('style');
  style.innerHTML = '*{ user-select: auto !important; }';

  document.body.appendChild(style);
})();

Note: This will apply to every page if left enabled, which might not be desirable in all situations.

The script can be installed and easily toggled on/off with a user script manager such as Violentmonkey, Tampermonkey, or Greasemonkey.