userChrome.css for Firefox to disable mute and play buttons in tabs

gistfile1.txt

/* Hide the mute/unmute button */
.tab-icon-overlay:not([pinned], [sharing], [crashed]):is([soundplaying], [muted], [activemedia-blocked]) {
    display: none !important;
}
/* Keep site icon visible on hover */
.tabbrowser-tab:hover .tab-icon-stack:not([pinned], [sharing], [crashed]):is([soundplaying], [muted], [activemedia-blocked]) > :not(.tab-icon-overlay), 
    /* for site icon with Compact density */
:root[uidensity="compact"] .tab-icon-stack:not([pinned], [sharing], [crashed]):is([soundplaying], [muted], [activemedia-blocked]) > :not(.tab-icon-overlay) {
    opacity: 1 !important; /* overrides full transparency with full opacity */
}