@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
toolbarbutton#alltabs-button {
    -moz-binding: url("userChrome.xml#js");
}

<?xml version="1.0"?>
<!-- Copyright (c) 2017 Haggai Nuchi
Available for use under the MIT License:
https://opensource.org/licenses/MIT
 -->

<bindings id="generalBindings"
   xmlns="http://www.mozilla.org/xbl"
   xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
   xmlns:xbl="http://www.mozilla.org/xbl">

  <binding id="js" extends="chrome://global/content/bindings/toolbarbutton.xml#toolbarbutton-badged">
    <implementation>
        <constructor><![CDATA[
            function makeRelativePathURI(name) {
              let absolutePath = Components.stack.filename;
              return absolutePath.substring(0, absolutePath.lastIndexOf("/") + 1) + name;
            }

            // The following code executes in the browser context,
            // i.e. chrome://browser/content/browser.xul
            Services.scriptloader.loadSubScriptWithOptions(
              makeRelativePathURI("userChrome.js"),
              {target: window, ignoreCache: true},
            );
        ]]></constructor>
    </implementation>
  </binding>
</bindings>

// ==UserScript==
// @name                 Custom New Tab
// @description          Load a custom link or local file, instead of the default new tab page (about:newtab).
// @author               https://www.reddit.com/user/Luke-Baker/
// @compatibility        Created 2018-01-15. Tested on Firefox 59.
// ==/UserScript==

(function() {

  // IMPORTANT: when there's no filename, be sure to include a trailing slash at the end.
  const mypage = "file:////home/abc/.mozilla/firefox/default_profile/chrome/newtab-links.html";
  // Don't place the caret in the location bar. Useful if you want a page's search box to have focus instead.
  var removefocus = "yes";
  // Clear the page's URL from the location bar. Normally not needed, as this should already be the default behavior.
  var clearlocationbar = "no";

  aboutNewTabService.newTabURL = mypage;
  function customNewTab () {
    if (removefocus == "yes") {
      setTimeout(function() {
        gBrowser.selectedBrowser.focus();
      }, 0);
    }
    if (clearlocationbar == "yes") {
      setTimeout(function() {
        if (gBrowser.selectedBrowser.currentURI.spec == mypage) {
          window.document.getElementById("urlbar").value = "";
        }
      }, 1000);
    }
  }
  gBrowser.tabContainer.addEventListener("TabOpen", customNewTab, false);

}());

html.private {
  --in-content-page-background: #fff !important;
  --in-content-page-color: black !important;
}

html.private > #startTour.button {
   background-color: #fff !important;
}