_global.css

/* 
if the -moz-appearance: none; still gives you fullscreen issues 
change (#navigator-toolbox) with your top bar (in this case #nav-bar) and use 
-moz-appearance: -moz-window-titlebar-maximized !important; And remove the padding-top 
*/
:root[tabsintitlebar][sizemode="maximized"] #navigator-toolbox {
    -moz-appearance: none !important;
    padding-top: 12px !important;
}
/* Fix the FUllscreen changing padding on other bars. */
:root[tabsintitlebar][sizemode="maximized"] #titlebar {
    -moz-appearance: none !important;
}

/* Change bars order */
#PersonalToolbar {
    -moz-box-ordinal-group: 3 !important;
}
#titlebar {
    -moz-box-ordinal-group: 2 !important;
}
#nav-bar {
    -moz-box-ordinal-group: 1 !important;
}

/* Change bottom border color */
#navigator-toolbox {
    border-color: var(--moonlight-gray-1) !important;
    background-color: var(--moonlight-gray-2) !important;
    padding: 0 3px 3px 3px !important;
}

#tabbrowser-tabpanels {
    background-color: var(--moonlight-gray-2) !important;
}