UserChrome.css

~/Library/Application Support/Firefox/Profiles/hc82ixoy.default/chrome/userChrome.css

/* Tab bar below Navigation & Bookmarks Toolbars */
#navigator-toolbox {
    display: flex;
    flex-direction: column;
}

#nav-bar { /* main toolbar */
  -moz-box-ordinal-group: 1 !important;
  order: 1;
  box-shadow: none !important;
}
#PersonalToolbar { /* bookmarks toolbar */
  -moz-box-ordinal-group: 2 !important;
  order: 2;
}
#TabsToolbar {
    order: 3 !important;
    margin-top: 0 !important;
}

#titlebar { /* menu bar + tabs bar */
  -moz-box-ordinal-group: 3 !important;
  order: 3;
  padding-top: 3px !important;
  background-color:#fff;
}

#personal-bookmarks {margin:3px 0}
#personal-bookmarks .bookmark-item{padding-right:7px!important}

#tabbrowser-tabs {
    --user-tab-rounding: 4px;
}

.tab-background {
    border-radius: var(--user-tab-rounding) var(--user-tab-rounding) 0px 0px !important;
    margin-block: 1px 0 !important;
	background-color:#eee;
}
#scrollbutton-up, #scrollbutton-down { /* 6/10/2021 */
    border-top-width: 1px !important;
    border-bottom-width: 0 !important;
}
/* Container color bar visibility */
.tabbrowser-tab[usercontextid] > .tab-stack > .tab-background > .tab-context-line {
    margin: 0px max(calc(var(--user-tab-rounding) - 3px), 0px) !important;
}