funktioniert bei mir leider gar nicht
War ja auch falsch, um das Favicon bei soundplaying einzublenden, benötigst Du das ↓ Schnipsel.
CSS
:root[uidensity="compact"] .tab-icon-stack:not([pinned], [sharing], [crashed]):is([soundplaying], [muted], [activemedia-blocked]) > :not(.tab-icon-overlay), #tabbrowser-tabs[secondarytext-unsupported] .tab-icon-stack:not([pinned], [sharing], [crashed]):is([soundplaying], [muted], [activemedia-blocked]) > :not(.tab-icon-overlay),
:root:not([uidensity="compact"]) #tabbrowser-tabs:not([secondarytext-unsupported]) .tabbrowser-tab:not(:hover) .tab-icon-overlay:not([pinned], [sharing], [crashed]):is([soundplaying], [muted], [activemedia-blocked]),
:root:not([uidensity="compact"]) #tabbrowser-tabs:not([secondarytext-unsupported]) .tabbrowser-tab:hover .tab-icon-stack:not([pinned], [sharing], [crashed]):is([soundplaying], [muted], [activemedia-blocked]) > :not(.tab-icon-overlay) {
opacity: 1.0 !important;
}