urlの記述が誤っていたんじゃないかな元のcssでもいけた

:root {--my-favicon-size: 16px;}
.bookmark-item[scheme="https"] image,
.bookmark-item[scheme="https"] > :is(.menu-icon,.menu-iconic-icon),
.bookmark-item[scheme="http"] image,
.bookmark-item[scheme="http"] > :is(.menu-icon,.menu-iconic-icon) {
background: url("chrome://browser/skin/downloads/downloads.svg") center/var(--my-favicon-size) no-repeat !important;
padding-left: var(--my-favicon-size) !important;
}
:root {--my-favicon-size: 16px;}
.bookmark-item[scheme="javascript"] image,
.bookmark-item[scheme="javascript"] > :is(.menu-icon,.menu-iconic-icon) {
background: url("chrome://mozapps/skin/extensions/extension.svg") center/var(--my-favicon-size) no-repeat !important;
padding-left: var(--my-favicon-size) !important;
}