放到userChrome.css里的样式为何不起作用

https://www.firefox.net.cn/read-48077

/* Hides scrollbar */scrollbar[orient="vertical"]:not(:hover),
scrollbar[orient="horizontal"]:not(:hover) {
    opacity: 0 !important;
}
  
/* Hides scrollbar buttons */
scrollbar scrollbarbutton {
    visibility: collapse !important;
}
  
/* Removes whitespace and allows scrollbar to appear */
 scrollbar {
    position: relative !important;
    -moz-appearance: none !important;
    background: none !important;
    z-index: 9999 !important;
}
  
/* Appearance of scrollbar thumb */
 scrollbar thumb {
    position: relative !important;
    -moz-appearance: none !important;
    background: rgba(110,110,110, .6) !important;
    border: 0 !important;
}
  
/* Color of scrollbar thumb on mouseover */
 scrollbar thumb:hover {
    background: rgba(110,110,110,.8) !important;
}
  
/* Color of scrollbar when clicked */
 scrollbar thumb:active {
    background: rgba(110,110,110, 1) !important;
}
  
/* Scrollbar sizes */
 scrollbar[orient="vertical"] {
    margin-left: -10px !important;
}
 scrollbar[orient="horizontal"] {
    margin-top: -10px !important;
}

很久以前用样式扩展有效。但放到userChrome.css里从来没生效过。

滚动条已经不能用CSS更改了，只能用js更改。