https://userstyles.org/styles/99710/australis-calendar
Nettes Replacement der Menu-Waffel funktioniert leider seit heute nicht mehr in allen aktuellen Versionen des FF. Irgendetwas stimmt mit der Datums-Identifikation nicht. Gibt statt dem Datum und Monat Invalid Date aus.
Vielleicht kann einer der Script Gurus hier 
da mal reinschauen. Sieht nicht besonders kompliziert aus, aber irgendwo muß da ein Bock drin sein.
Teste das doch bitte mal:
@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
@-moz-document url(chrome://browser/content/browser.xul) {
/**
* Australis Calendar
* Licensed under the MIT license
*
* Copyright (c) 2014 LouCypher.
*
* Permission is hereby granted, free of charge, to any person obtaining a
* copy of this software and associated documentation files (the "Software"),
* to deal in the Software without restriction, including without limitation
* the rights to use, copy, modify, merge, publish, distribute, sublicense,
* and/or sell copies of the Software, and to permit persons to whom the
* Software is furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
* DEALINGS IN THE SOFTWARE.
**/
#PanelUI-menu-button .toolbarbutton-icon,
#PanelUI-menu-button .toolbarbutton-badge-stack {
visibility: collapse;
}
#PanelUI-menu-button::before,
#PanelUI-menu-button::after {
display: -moz-box;
}
#PanelUI-menu-button:not([open]):hover::before,
#PanelUI-menu-button:not([open]):hover::after {
background-image: linear-gradient(rgba(255, 255, 255, .6),
rgba(255, 255, 255, .1));
background-color: transparent;
border-color: hsla(210, 54%, 20%, .15) hsla(210, 54%, 20%, .2) hsla(210, 54%, 20%, .25);
box-shadow: 0 1px hsla(0, 0%, 100%, .3) inset,
0 1px hsla(210, 54%, 20%, .03),
0 0 2px hsla(210, 54%, 20%, .1);
}
#PanelUI-menu-button[open]::before,
#PanelUI-menu-button[open]::after {
background-color: hsla(210, 4%, 10%, .12);
border-top-color: hsla(210, 4%, 10%, .2);
box-shadow: 0 1px 0 0 hsla(210, 4%, 10%, .1) inset;
transition-duration: 10ms;
}
#PanelUI-menu-button::before {
padding: .5em .25em .5em .5em;
}
#PanelUI-menu-button::after {
padding: .5em .5em .5em .25em;
}
:root[stylish-month="1"] #PanelUI-menu-button::before {
content: "Jan";
}
:root[stylish-month="2"] #PanelUI-menu-button::before {
content: "Feb";
}
:root[stylish-month="3"] #PanelUI-menu-button::before {
content: "Mar";
}
:root[stylish-month="4"] #PanelUI-menu-button::before {
content: "Apr";
}
:root[stylish-month="5"] #PanelUI-menu-button::before {
content: "May";
}
:root[stylish-month="6"] #PanelUI-menu-button::before {
content: "Jun";
}
:root[stylish-month="7"] #PanelUI-menu-button::before {
content: "Jul";
}
:root[stylish-month="8"] #PanelUI-menu-button::before {
content: "Aug";
}
:root[stylish-month="9"] #PanelUI-menu-button::before {
content: "Sep";
}
:root[stylish-month="10"] #PanelUI-menu-button::before {
content: "Oct";
}
:root[stylish-month="11"] #PanelUI-menu-button::before {
content: "Nov";
}
:root[stylish-month="12"] #PanelUI-menu-button::before {
content: "Dec";
}
:root[stylish-date="1"] #PanelUI-menu-button::after {
content: "1";
}
:root[stylish-date="2"] #PanelUI-menu-button::after {
content: "2";
}
:root[stylish-date="3"] #PanelUI-menu-button::after {
content: "3";
}
:root[stylish-date="4"] #PanelUI-menu-button::after {
content: "4";
}
:root[stylish-date="5"] #PanelUI-menu-button::after {
content: "5";
}
:root[stylish-date="6"] #PanelUI-menu-button::after {
content: "6";
}
:root[stylish-date="7"] #PanelUI-menu-button::after {
content: "7";
}
:root[stylish-date="8"] #PanelUI-menu-button::after {
content: "8";
}
:root[stylish-date="9"] #PanelUI-menu-button::after {
content: "9";
}
:root[stylish-date="10"] #PanelUI-menu-button::after {
content: "10";
}
:root[stylish-date="11"] #PanelUI-menu-button::after {
content: "11";
}
:root[stylish-date="12"] #PanelUI-menu-button::after {
content: "12";
}
:root[stylish-date="13"] #PanelUI-menu-button::after {
content: "13";
}
:root[stylish-date="14"] #PanelUI-menu-button::after {
content: "14";
}
:root[stylish-date="15"] #PanelUI-menu-button::after {
content: "15";
}
:root[stylish-date="16"] #PanelUI-menu-button::after {
content: "16";
}
:root[stylish-date="17"] #PanelUI-menu-button::after {
content: "17";
}
:root[stylish-date="18"] #PanelUI-menu-button::after {
content: "18";
}
:root[stylish-date="19"] #PanelUI-menu-button::after {
content: "19";
}
:root[stylish-date="20"] #PanelUI-menu-button::after {
content: "20";
}
:root[stylish-date="21"] #PanelUI-menu-button::after {
content: "21";
}
:root[stylish-date="22"] #PanelUI-menu-button::after {
content: "22";
}
:root[stylish-date="23"] #PanelUI-menu-button::after {
content: "23";
}
:root[stylish-date="24"] #PanelUI-menu-button::after {
content: "24";
}
:root[stylish-date="25"] #PanelUI-menu-button::after {
content: "25";
}
:root[stylish-date="26"] #PanelUI-menu-button::after {
content: "26";
}
:root[stylish-date="27"] #PanelUI-menu-button::after {
content: "27";
}
:root[stylish-date="28"] #PanelUI-menu-button::after {
content: "28";
}
:root[stylish-date="29"] #PanelUI-menu-button::after {
content: "29";
}
:root[stylish-date="30"] #PanelUI-menu-button::after {
content: "30";
}
:root[stylish-date="31"] #PanelUI-menu-button::after {
content: "31";
}
}Hat sich erledigt nach einem Reboot 
muß irgendwas im W10 quer gelegen haben im Abgriff der Systen-Zeit. Aber irritierend war es schon, hatte erst den letzten Update von CTR im Verdacht, aber der war noch nicht überall drauf.
Trotzdem vilen Dank für Deien Mühe