From https://github.com/nuchi/firefox-quantum-userchromejs ¿?

FloatingScrollbar.uc.js

// ==UserScript==
// @name           FloatingScrollbar.uc.js
// @namespace      nightson1988@gmail.com
// @include        main
// @version        0.0.3
// @note           Thanks to Griever(https://github.com/Griever/userChromeJS/blob/master/SmartScrollbar.uc.js) and Paul Rouget(https://gist.github.com/4003205)
// @note...........0.0.3 Fixed a problem of breaking hbox layout 
// @note           0.0.2 Remove usage of E4X (https://bugzilla.mozilla.org/show_bug.cgi?id=788293)
// ==/UserScript==

(function () {
    var prefs = Services.prefs,
        enabled;
    if (prefs.prefHasUserValue('userChromeJS.floating_scrollbar.enabled')) {
        enabled = prefs.getBoolPref('userChromeJS.floating_scrollbar.enabled')
    } else {
        prefs.setBoolPref('userChromeJS.floating_scrollbar.enabled', true);
        enabled = true;
    }

    var css = '\
    @namespace url(http: //www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);\
    :not(select):not(hbox) > scrollbar {\
        -moz-appearance: none!important;\
        position: relative;\
        background-color: transparent;\
        background-image: none;\
        z-index: 2147483647;\
        padding: 2px;\
    }\
    :not(select):not(hbox) > scrollbar[orient = "vertical"] {\
        -moz-margin-start: -10px;\
        min-width: 10px;\
    }\
    :not(select):not(hbox) > scrollbar[orient = "vertical"] thumb {\
        min-height: 20px;\
    }\
   :not(select):not(hbox) > scrollbar[orient = "horizontal"] {\
        margin-top: -10px;\
        min-height: 10px;\
    }\
    :not(select):not(hbox) > scrollbar[orient = "horizontal"] thumb {\
        min-width: 20px;\
    }\
    :not(select):not(hbox) > scrollbar thumb {\
        -moz-appearance: none!important;\
        border-width: 0px!important;\
        border-radius: 3px!important;\
        background-color: rgba(0, 0, 0, 0.1)!important;\
        transition: background-color .3s ease-in-out;\
    }\
    :not(select):not(hbox) > scrollbar thumb:active,\
    :not(select):not(hbox) > scrollbar thumb:hover {\
        background-color: #9B9B9B!important;\
    }\
    :not(select):not(hbox) > scrollbar scrollbarbutton, :not(select):not(hbox) > scrollbar gripper {\
        display: none;\
    }';

    var sss = Cc['@mozilla.org/content/style-sheet-service;1'].getService(Ci.nsIStyleSheetService);
    var uri = makeURI('data:text/css;charset=UTF=8,' + encodeURIComponent(css));

    var p = document.getElementById('devToolsSeparator');
    var m = document.createElement('menuitem');
    m.setAttribute('label', "Schwebende Scrollbar");
    m.setAttribute('type', 'checkbox');
    m.setAttribute('autocheck', 'false');
    m.setAttribute('checked', enabled);
    p.parentNode.insertBefore(m, p);
    m.addEventListener('command', command, false);

    if (enabled) {
        sss.loadAndRegisterSheet(uri, sss.AGENT_SHEET);
    }

    function command() {
        if (sss.sheetRegistered(uri, sss.AGENT_SHEET)) {
            prefs.setBoolPref('userChromeJS.floating_scrollbar.enabled', false);
            sss.unregisterSheet(uri, sss.AGENT_SHEET);
            m.setAttribute('checked', false);
        } else {
            prefs.setBoolPref('userChromeJS.floating_scrollbar.enabled', true);
            sss.loadAndRegisterSheet(uri, sss.AGENT_SHEET);
            m.setAttribute('checked', true);
        }

        let root = document.documentElement;
        let display = root.style.display;
        root.style.display = 'none';
        window.getComputedStyle(root).display; // Flush
        root.style.display = display;
    }

})();

userChrome.css

@namespace html url("http://www.w3.org/1999/xhtml");
@namespace svg url("http://www.w3.org/2000/svg");
@-moz-document url(chrome://browser/content/browser.xul) {
#alltabs-button {
	-moz-binding: url(data:text/xml;base64,PD94bWwgdmVyc2lvbj0iMS4wIj8+DQo8IS0tIENvcHlyaWdodCAoYykgMjAxNyBIYWdnYWkgTnVjaGkNCkF2YWlsYWJsZSBmb3IgdXNlIHVuZGVyIHRoZSBNSVQgTGljZW5zZToNCmh0dHBzOi8vb3BlbnNvdXJjZS5vcmcvbGljZW5zZXMvTUlUDQogLS0+DQoNCjwhLS0gUnVuIHVzZXJDaHJvbWUuanMvdXNlckNocm9tZS54dWwgYW5kIC51Yy5qcy8udWMueHVsLy5jc3MgZmlsZXMgIC0tPg0KPGJpbmRpbmdzIHhtbG5zPSJodHRwOi8vd3d3Lm1vemlsbGEub3JnL3hibCI+DQogICAgPGJpbmRpbmcgaWQ9ImpzIiBleHRlbmRzPSJjaHJvbWU6Ly9nbG9iYWwvY29udGVudC9iaW5kaW5ncy90b29sYmFyYnV0dG9uLnhtbCNtZW51Ij4NCiAgICAgICAgPGltcGxlbWVudGF0aW9uPg0KICAgICAgICAgICAgPGNvbnN0cnVjdG9yPjwhW0NEQVRBWw0KICAgICAgICAgICAgICAgIGlmKHdpbmRvdy51c2VyQ2hyb21lSnNNb2QpIHJldHVybjsNCiAgICAgICAgICAgICAgICB3aW5kb3cudXNlckNocm9tZUpzTW9kID0gdHJ1ZTsNCiAgICAgICAgICAgICAgICANCiAgICAgICAgICAgICAgICB2YXIgY2hyb21lRmlsZXMgPSBGaWxlVXRpbHMuZ2V0RGlyKCJVQ2hybSIsIFtdKS5kaXJlY3RvcnlFbnRyaWVzOw0KICAgICAgICAgICAgICAgIHZhciB4dWxGaWxlcyA9IFtdOw0KICAgICAgICAgICAgICAgIHZhciBzc3MgPSBDY1snQG1vemlsbGEub3JnL2NvbnRlbnQvc3R5bGUtc2hlZXQtc2VydmljZTsxJ10uZ2V0U2VydmljZShDaS5uc0lTdHlsZVNoZWV0U2VydmljZSk7DQogICAgICAgICAgICAgICAgDQogICAgICAgICAgICAgICAgd2hpbGUoY2hyb21lRmlsZXMuaGFzTW9yZUVsZW1lbnRzKCkpIHsNCiAgICAgICAgICAgICAgICAgICAgdmFyIGZpbGUgPSBjaHJvbWVGaWxlcy5nZXROZXh0KCkuUXVlcnlJbnRlcmZhY2UoQ2kubnNJRmlsZSk7DQogICAgICAgICAgICAgICAgICAgIHZhciBmaWxlVVJJID0gU2VydmljZXMuaW8ubmV3RmlsZVVSSShmaWxlKTsNCiAgICAgICAgICAgICAgICAgICAgDQogICAgICAgICAgICAgICAgICAgIGlmKGZpbGUuaXNGaWxlKCkpIHsNCiAgICAgICAgICAgICAgICAgICAgICAgIGlmKC8oXnVzZXJDaHJvbWV8XC51YylcLmpzJC9pLnRlc3QoZmlsZS5sZWFmTmFtZSkpIHsNCiAgICAgICAgICAgICAgICAgICAgICAgICAgICBTZXJ2aWNlcy5zY3JpcHRsb2FkZXIubG9hZFN1YlNjcmlwdFdpdGhPcHRpb25zKGZpbGVVUkkuc3BlYywge3RhcmdldDogd2luZG93LCBpZ25vcmVDYWNoZTogdHJ1ZX0pOw0KICAgICAgICAgICAgICAgICAgICAgICAgfQ0KICAgICAgICAgICAgICAgICAgICAgICAgZWxzZSBpZigvKF51c2VyQ2hyb21lfFwudWMpXC54dWwkL2kudGVzdChmaWxlLmxlYWZOYW1lKSkgew0KICAgICAgICAgICAgICAgICAgICAgICAgICAgIHh1bEZpbGVzLnB1c2goZmlsZVVSSS5zcGVjKTsNCiAgICAgICAgICAgICAgICAgICAgICAgIH0NCiAgICAgICAgICAgICAgICAgICAgICAgIGVsc2UgaWYoL1wuYXNcLmNzcyQvaS50ZXN0KGZpbGUubGVhZk5hbWUpKSB7DQogICAgICAgICAgICAgICAgICAgICAgICAgICAgaWYoIXNzcy5zaGVldFJlZ2lzdGVyZWQoZmlsZVVSSSwgc3NzLkFHRU5UX1NIRUVUKSkNCiAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgc3NzLmxvYWRBbmRSZWdpc3RlclNoZWV0KGZpbGVVUkksIHNzcy5BR0VOVF9TSEVFVCk7DQogICAgICAgICAgICAgICAgICAgICAgICB9DQogICAgICAgICAgICAgICAgICAgICAgICBlbHNlIGlmKC9eKD8hKHVzZXJDaHJvbWV8dXNlckNvbnRlbnQpXC5jc3MkKS4rXC5jc3MkL2kudGVzdChmaWxlLmxlYWZOYW1lKSkgew0KICAgICAgICAgICAgICAgICAgICAgICAgICAgIGlmKCFzc3Muc2hlZXRSZWdpc3RlcmVkKGZpbGVVUkksIHNzcy5VU0VSX1NIRUVUKSkNCiAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgc3NzLmxvYWRBbmRSZWdpc3RlclNoZWV0KGZpbGVVUkksIHNzcy5VU0VSX1NIRUVUKTsNCiAgICAgICAgICAgICAgICAgICAgICAgIH0NCiAgICAgICAgICAgICAgICAgICAgfQ0KICAgICAgICAgICAgICAgIH0NCiAgICAgICAgICAgICAgICANCiAgICAgICAgICAgICAgICBzZXRUaW1lb3V0KGZ1bmN0aW9uIGxvYWRYVUwoKSB7DQogICAgICAgICAgICAgICAgICAgIGlmKHh1bEZpbGVzLmxlbmd0aCA+IDApIHsNCiAgICAgICAgICAgICAgICAgICAgICAgIGRvY3VtZW50LmxvYWRPdmVybGF5KHh1bEZpbGVzLnNoaWZ0KCksIG51bGwpOw0KICAgICAgICAgICAgICAgICAgICAgICAgc2V0VGltZW91dChsb2FkWFVMLCA1KTsNCiAgICAgICAgICAgICAgICAgICAgfQ0KICAgICAgICAgICAgICAgIH0sIDApOw0KICAgICAgICAgICAgXV0+PC9jb25zdHJ1Y3Rvcj4NCiAgICAgICAgPC9pbXBsZW1lbnRhdGlvbj4NCiAgICA8L2JpbmRpbmc+DQo8L2JpbmRpbmdzPg==);
}
}