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

The code is based on the fantastic answer from Brock Adams - all praise should be his!

// ==UserScript==
// @name        StackExchange, Expand All Comments
// @author       Taken from https://webapps.stackexchange.com/a/119439/192282
// @include      https://*stackoverflow.com/*
// @include      https://*serverfault.com/*
// @include      https://*superuser.com/*
// @include      https://*askubuntu.com/*
// @include      https://*mathoverflow.net/*
// @include      https://*.stackexchange.com/*
// @grant       none
// ==/UserScript==

try {
//--- This is a utility function that SE currently provides on its pages.
StackExchange.ready (function () {
$('.js-show-link').trigger ('click');
} );
}
catch (e) {
console.warn ("***Userscript error: StackExchange.ready() is no longer defined!");
}