userChrome.css

/*
1. Place this file  in <profile dir>/chrome/userChrome.css (about:support)
2. Enable `toolkit.legacyUserProfileCustomizations.stylesheets` (about:config)
3. Source https://amitp.blogspot.com/2021/06/firefox-89-tab-appearance.html?m=1
*/

.tab-background {
    background: #585060;
    border-radius: 5px 5px 0 0 !important;
    margin-bottom: 0px !important;
    border: 1px solid rgba(0,0,0,.3) !important;
    border-bottom-width: 0px !important;
}

#TabsToolbar {
    background: #585060;
    color: #ffffff;
}

.tab-background:not([selected]) {
    background-color: #666 !important;
    color: #ffffff;
}