macos-native-tabbar.css

/* ------------------------------------ */
/* Firefox Translucent Tabbar for macOS */
/* ------------------------------------ */

:root#main-window {
   background-color: transparent !important;
}

:root:not(:-moz-window-inactive) #navigator-toolbox {
  background-color: transparent !important; 
}

/* make tabs toolbar translucent */
:root #TabsToolbar,
:root #titlebar,
:root #tabbrowser-tabs {
   -moz-default-appearance: -moz-window-titlebar !important;
   appearance: -moz-window-titlebar !important;
   background-color: transparent !important;
}

/* De-emphasize tab icons and names when window is unfocused */
:root:-moz-window-inactive .tab-content {
  opacity: 0.6;
}

/* recolor toolbar button hover/active bg colors to semi-transparent */
:root {
   --toolbarbutton-hover-background: color-mix(in srgb, currentColor 11%, transparent) !important;
   --toolbarbutton-active-background: color-mix(in srgb, currentColor 21%, transparent) !important;
}

/* improve border visiblity for selected tabs */
.tab-background:is([selected], [multiselected]):-moz-lwtheme {
   border:  none !important;
}
#tabbrowser-tabs:not([noshadowfortests]) .tab-background:is([selected], [multiselected]) {
   box-shadow: 0 0 0 var(--tabs-navbar-shadow-size) rgba(0,0,0,.1), 0 1px 5px rgba(0,0,0,.1) !important;
}
:root:-moz-window-inactive #tabbrowser-tabs:not([noshadowfortests]) .tab-background:is([selected], [multiselected]) {
   box-shadow: 0 0 0 var(--tabs-navbar-shadow-size) rgba(0,0,0,.05) !important;
}

@media (-moz-content-prefers-color-scheme: dark) {
  :root #tabbrowser-tabs:not([noshadowfortests]) .tab-background:is([selected], [multiselected]) {
     box-shadow: 0 0 0 var(--tabs-navbar-shadow-size) hsla(280,6%,60%,0.5) inset, 0 0 0 var(--tabs-navbar-shadow-size) rgba(0,0,0,.6), 0 1px 5px rgba(0,0,0,.1) !important;
  }
  :root:-moz-window-inactive #tabbrowser-tabs:not([noshadowfortests]) .tab-background:is([selected], [multiselected]) {
     box-shadow: 0 0 0 var(--tabs-navbar-shadow-size) rgba(0,0,0,.4) !important;
  }
}

/* border between tab bar and navbar */
:root #nav-bar {
  box-shadow: 0 -1px 0 0 hsla(280,6%,0%,0.1), 0 -4px 4px -5px hsla(280,6%,0%,0.2);
}

@media (-moz-content-prefers-color-scheme: dark) {
  :root #nav-bar {
    box-shadow: 0 -1px 0 0 hsla(280,6%,0%,0.3), 0 -4px 4px -5px hsla(280,6%,0%,0.05) !important;
  }
}