Bewerken van UserChrome.css

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

/* Tab bar below Navigation & Bookmarks Toolbars */

    nav-bar { /* main toolbar */

 -moz-box-ordinal-group: 1 !important;
 box-shadow: none !important;

}

    PersonalToolbar { /* bookmarks toolbar */

 -moz-box-ordinal-group: 2 !important;

}

    titlebar { /* menu bar + tabs bar */

 -moz-box-ordinal-group: 3 !important;
 padding-top: 3px !important;
 background-color:#fff;

}

    personal-bookmarks {margin:3px 0}
    personal-bookmarks .bookmark-item{padding-right:8px!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;

} 
</code>