demo. very incomplete.

highlightScrollbar.uc.js

// ==UserScript==
// @name           highlightScrollbar.uc.js
// @include        main
// ==/UserScript==

(function() {

  messageManager.loadFrameScript('data:application/javascript;charset=UTF-8,' + 
                  encodeURIComponent('('+(function(){
    var dis = this;
    ChromeUtils.import("resource://gre/modules/Finder.jsm", this);
    ChromeUtils.import("resource://gre/modules/FindContent.jsm", this);
    
    this.fbtMandaAltura = function () {
      var list = [];
      var size = content.document.documentElement.getBoundingClientRect().height;
      var spacobar = content.document.documentElement.clientHeight - 34;
      hi._collectRectData().forEach(res => {
        list.push((res.rectsAndTexts.rectList[0].top / size) * spacobar + 17 + 'px');
      })
      sendAsyncMessage('fbt', list);
    }
    this.Finder.prototype.onFindbarClose = function onFindbarClose() {
      this.enableSelection();
      this.highlighter.highlight(false);
      sendAsyncMessage('fbtEsconde');
      this.iterator.reset();
      BrowserUtils.trackToolbarVisibility(this._docShell, "findbar", false);
    }
    this.Finder.prototype._notifyMatchesCount = function _notifyMatchesCount(result = this._currentMatchesCountResult) {
      // The `_currentFound` property is only used for internal bookkeeping.
      delete result._currentFound;
      result.limit = this.matchesCountLimit;
      if (result.total == result.limit)
        result.total = -1;

      for (let l of this._listeners) {
        try {
          l.onMatchesCountResult(result);
        } catch (ex) {}
      }

      if (result.total && result.total < 101) {
        dis.hi = (new dis.FindContent(docShell));
        fbtMandaAltura();
      } else {
        sendAsyncMessage('fbtEsconde');
      }

      this._currentMatchesCountResult = null;
    }

    this.fbtTimeout = false;
    this.fbtResizou = function() {
      dis.hi = (new dis.FindContent(docShell));
      if (dis.hi._collectRectData().length) {
        fbtMandaAltura();
      }
    }
    addEventListener('DOMWindowCreated', function () {
      content.addEventListener('resize', function() {
        clearTimeout(fbtTimeout);
        fbtTimeout = setTimeout(fbtResizou, 100);
      });
    }, false);
  }).toString() + ')();'), true);

  messageManager.addMessageListener('fbt', function (message) {
    if (!gBrowser.selectedBrowser.fbt) {
      var gB = document.createElement('hbox');
      gB.id = 'gridbox';
      gB.style.direction = 'rtl';
      gB.style.pointerEvents = 'none';

      var fG = document.createElement('vbox');

      var hl = document.createElement('vbox');
      hl.id = 'fbtV';
      hl.flex = '1';
      hl.style.position = 'relative';
      hl.style.padding = '17px 1px 17px 0px';

      fG.appendChild(hl);
      gBrowser.selectedBrowser.fbt = hl;

      gB.appendChild(fG);

      var sss = Components.classes["@mozilla.org/content/style-sheet-service;1"];

      var estilo = '#fbtV > vbox { height: 2px; width: 15px; background-color: #38d878; z-index: 100; position: absolute; left: -15px; right: 1px; }';

      sss.getService(Ci.nsIStyleSheetService).loadAndRegisterSheet(Cc["@mozilla.org/network/io-service;1"].getService(Ci.nsIIOService).newURI('data:text/css;charset=UTF-8,' + encodeURIComponent(estilo), null, null), sss.AGENT_SHEET);

      for(let i = 0; i < 100; i++) {
        gBrowser.selectedBrowser.fbt.appendChild(document.createElement('vbox'));
      }

      gBrowser.selectedBrowser.parentNode.appendChild(gB);
    }

    for (let i = 0; i < 100; i++) {
      if (i < message.data.length) {
        gBrowser.selectedBrowser.fbt.childNodes[i].style.top = message.data[i];
        gBrowser.selectedBrowser.fbt.childNodes[i].style.display = 'block';
      } else {
        gBrowser.selectedBrowser.fbt.childNodes[i].style.display = 'none';
      }
    }
  });

  messageManager.addMessageListener('fbtEsconde', function () {
    gBrowser.selectedBrowser.fbt && gBrowser.selectedBrowser.fbt.childNodes.forEach(el => el.style.display = 'none');
  });

})();