#main-window[sizemode="normal"]{ border-top: none !important }
#navigator-toolbox::after{ content: none !important }

/* OPTIONAL - uncomment to remove still remaining space atop of tabs */
/* This just increases the height of tabs by few pixels, not decrease the toolbar height */

/* Fx pre-65 */
/* #navigator-toolbox > #TabsToolbar{margin-top: -2px;} */
/* Fx65+ */
/* #TabsToolbar > .toolbar-items > spacer{ display: none; } */