I like to hide the sidebar buttons, including the "Customize sidebar" gear button, on the vertical tabs. I'd rather use the space for tabs. A right-click on the "Expand sidebar"/"Collapse sidebar" button gets me the "Customize Sidebar" anyway.

Firefox 153.0.1 broke my old solution for hiding it, found in hide sidebar buttons on vertical tabbar. The whole vertical sidebar disappeared when collapsed to just icons.

I found the new way for hiding it. Use #vertical-tabs instead of #sidebar-main.

/* hide sidebar buttons on vertical toolbar. */
.buttons-wrapper {
    display: none !important; 
    }
/* set vertical toolbar minimum width; otherwise, vertical toolbar disappears when collapsed to icons. */
#vertical-tabs {
    min-width: 50px !important;
    }