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

The code is based on the fantastic, but unfortunately deleted, answer from Brock Adams (screenshot for users below 10k) - 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);
}

Known issues:

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