Stop Facebook from maxing out your video volume. Slightly modified version of the original script by Daile Alimo: https://gist.github.com/JustDaile/478a4e1e07003648865984ede307093d. Modified to use local storage since I couldn't get it to work using cookies. Tested with Firefox 57 and Violentmonkey 2.8.24.

facebook_volume.user.js

// ==UserScript==
// @name         FBVVM
// @namespace    http://tampermonkey.net/
// @version      1.1
// @description  Maintains set volume of videos on play of native video links on Facebook.
// @author       Daile Alimo, pezcode
// @match        https://www.facebook.com/*
// @grant        none
// @require      http://code.jquery.com/jquery-latest.js
// ==/UserScript==

(function() {
    'use strict';
    
    var volume = parseFloat(localStorage.FBVVM_volume) || 1;
    
    // scan for new videos
    setInterval(function() {
        var $scanning = $('video:not(.detected)');
        $scanning.each(function() {
            var $elem = $(this).addClass("detected");
            $elem.on('play', function() {
                this.volume = volume;
            });
            $elem.extend({
                ready : false,
            });
            $elem.on('volumechange', function() {
                var $this = $(this);
                if($this.ready) {
                    volume = this.volume;
                    localStorage.FBVVM_volume = volume;
                } else {
                    $this.ready = true;
                }
            });
        });
    }, 200);
})();