"Download any YouTube video from anywhere" script is a User Scrip for YouTube that will allow you to download videos as offline files from YouTube.com. This script will work best in Firefox 42. It can be installed in Firefox using the Greasemonkey add-on. In "about:config", media source streaming options should also be disabled.

com.vsubhash.js.youtube-downloader

// ==UserScript==
// @name        YouTube Downloader JavaScript
// @namespace   com.vsubhash.js.youtube-downloader
// @description Downloads YouTube videos
// @include     https://www.youtube.com/*
// @version     2018
// @grant       none
// ==/UserScript==


/*
 * Moral Volcano's YouTub DL JavaScript
 * Licensed under GPL v3 as provided at
 * https://www.gnu.org/licenses/gpl.html
 *
 */

document.addEventListener("DOMContentLoaded", addYouTubeDetectingButton, false);

function parseYTPlayer() {
  console.error("Inside parser" );
  console.error("Title" + ytplayer.config.args.title);
  console.error("Format" + unescape(ytplayer.config.args.adaptive_fmts));
  
  try { 
    if (document.getElementsByTagName("video")[0] && (document.getElementsByTagName("video")[0].pause)) {
      document.getElementsByTagName("video")[0].pause();
    }
  } catch (e) {
    window.alert(e);
  }
  
  var arFormatParams, arFormats, i, j, sURL, sQuality, sMimeType, sExtension;
  var oSelect, oOption;
  oSelect = document.getElementById("mvyJsVideoDownloadList");
  
  if (oSelect == null) { 
    return;
  } else {
    // Reset download list
    oSelect.length = 0;    
  }
  
  
  arFormats = ytplayer.config.args.url_encoded_fmt_stream_map.split(",");
  for (i = 0; i < arFormats.length; i++) {
    //console.error("Format " + i + ". " + arFormats[i]);
    
    arFormatParams = arFormats[i].split("&")
    sURL = "";
    for (j = 0; j < arFormatParams.length; j++) {
      // console.error("\tFormat parameter " + arFormatParams[j]);
      if (arFormatParams[j].indexOf("url=") > -1) {
        sURL = unescape(arFormatParams[j].substring("url=".length));
      } else {
        if (arFormatParams[j].indexOf("type=") > -1) {
          sMimeType = unescape(arFormatParams[j].substring("type=".length));
        }
        
        if (arFormatParams[j].indexOf("quality=") > -1) {
          sQuality = arFormatParams[j].substring("quality=".length);
        }
      } 
    }
    
    if ((sURL.length > 0) && (sQuality.length > 0) && (sMimeType.length > 0)) {
      oOption = document.createElement("option");
      oOption.text = sQuality + " - " + sMimeType;
      sURL = sURL + 
             "&ptk=youtube_none&pltype=contentugc" +
             "&title=" + 
             escape(ytplayer.config.args.title);
      //console.error("Final URL " + sURL);       
      oOption.setAttribute("onclick", "(function() { location.href = '" + sURL + "'; })();");       
      oSelect.add(oOption);
    }
  }
  
  if (oSelect.length > 0) {
    oSelect.style.display = "inline";
  } else {
    oSelect.style.display = "none";
    window.alert("Some error had occurred");
  }
  
} 


function handle_DownloadButtonClick() {
  if (document.getElementsByTagName("video").length > 0) {
   if (document.getElementsByTagName("video")[0].src.length > 10) {
     document.getElementById("mvJsLink").setAttribute("href",document.getElementsByTagName("video")[0].src + "&title=" + escape(document.title));
     
     document.getElementById("mvJsLink").innerHTML = document.getElementsByTagName("video")[0].src.substring(0,35) + "...";
     document.getElementById("mvJsLink").click();
     
   } else {
     window.alert("First play the video a bit and pause. Then, we will get the video link.");
   }
  } else {
    window.alert("No video has loaded yet.");
  }
}




function addYouTubeDetectingButton() {
  console.log("Executing YouTube detector");
  var i, n, oDlButtonEl, oDlDiv, oVideosList;
  
  if ((location.href.indexOf("youtube.com/embed") != -1) ||
      (location.href.indexOf("youtube.com/watch") != -1)) {
    console.log("Inside YouTube Iframe");
    
    oDlDiv = document.createElement("div");
    oDlDiv.setAttribute("id", "mvyJsDiv");
    oDlDiv.setAttribute("style", "color: black; padding-left: 1em; background-color: orange!important; width: 100%!important; height: 1.2cm!important; font-size: 0.34cm!important; line-height: 0.4cm!important; font-family: sans-serif!important;  ");
    
    if (location.href.indexOf("youtube.com/embed") > -1) {
      oDlButtonEl = document.createElement("input");
      oDlDiv.setAttribute("id", "mvJsButton");    
      oDlButtonEl.setAttribute("type", "button");
      oDlButtonEl.setAttribute("value", "Grab Video");     

      oDlButtonEl.addEventListener("click", handle_DownloadButtonClick, false);
      
      oDlButtonEl.setAttribute("style", "color: white!important; border: 1px solid orange; box-shadow: 2px 2px 5px black; background-color: red!important; height: 0.9cm!important; width: 5cm!important; font-size: 0.4cm!important; line-height: 0.5cm!important; ; font-family: sans-serif!important;");
      oDlDiv.appendChild(oDlButtonEl);
      
      oVidLink = document.createElement("a");
      oVidLink.setAttribute("id", "mvJsLink");        
      oVidLink.setAttribute("style", "color: darkblue!important; text-decoration: underline!important; font-size: 0.3cm!important; margin: 0.2cm 1cm 0.2cm 1cm!important;");
      oVidLink.innerHTML="Video URL";
      oVidLink.setAttribute("href","javascript:window.alert('First play the video a bit and pause. Then, we will get the video link.');");
      oDlDiv.appendChild(oVidLink);
      document.getElementById("player").parentNode.insertBefore(oDlDiv, document.getElementById("player"));
    } else {
      oDlDiv.innerHTML = "Download video from:";
      oVideosList = document.createElement("select");
      oVideosList.setAttribute("id", "mvyJsVideoDownloadList");                
      oVideosList.setAttribute("style", "display: none; font-size: 0.3cm!important; margin: 0.2cm 1cm 0.2cm 1em!important;");
      oDlDiv.appendChild(oVideosList);
      document.getElementById("player").parentNode.insertBefore(oDlDiv, document.getElementById("player"));
      parseYTPlayer();  
    }
    
    document.getElementById("player").setAttribute("style", document.getElementById("player").getAttribute("style") + "height: 90%!important; ");      
    document.getElementsByTagName("body")[0].style="background-color: white!important; line-height: 0.5cm;";
    document.getElementById("player").parentNode.style="background-color: white!important; ";

  }
}

On YouTube pages:

On other pages: