The following script for Tampermonkey (multi browser) or Greasemonkey (Firefox) automatically expands all comments which are normally hidden behind the "show ... more comments" link: 

[![pic of "show more comments" link][1]][1]
    
The code is based on the [fantastic answer from Brock Adams][2] - Thank you! 

    // ==UserScript==
    // @name        StackExchange, Expand All Comments
    // @author      Taken from https://webapps.stackexchange.com/a/119439/192282
    // @description Automatically shows all comments for questions and answers
    // @match       *://*.askubuntu.com/*
    // @match       *://*.mathoverflow.net/*
    // @match       *://*.serverfault.com/*
    // @match       *://*.stackapps.com/*
    // @match       *://*.stackexchange.com/*
    // @match       *://*.stackoverflow.com/*
    // @match       *://*.superuser.com/*
    // @exclude     *://api.stackexchange.com/*
    // @exclude     *://blog.stackexchange.com/*
    // @exclude     *://blog.stackoverflow.com/*
    // @exclude     *://chat.stackexchange.com/*
    // @exclude     *://chat.stackoverflow.com/*
    // @exclude     *://data.stackexchange.com/*
    // @exclude     *://elections.stackexchange.com/*
    // @exclude     *://openid.stackexchange.com/*
    // @exclude     *://stackexchange.com/*
    // @exclude     *://*/review
    // @grant       none
    // @version     1.1
    // @history     1.1 Avoid clicking on disabled "add a comment" links.
    // @history     1.0 Initial release.
    // ==/UserScript==

    try {
        //--- This is a utility function that SE currently provides on its pages.
        StackExchange.ready (function () {
            $('.js-show-link').not (".dno").trigger ('click');
        } );
    }
    catch (e) {
        console.error ("***Userscript error: StackExchange.ready() is no longer defined!", e);
    }

<br>

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Known issues:

 - May not expand comments if cookies are disabled (side effect of SE's anonymous user handling).

  [1]: https://i.sstatic.net/NsQWL.png
  [2]: https://webapps.stackexchange.com/a/119439/192282