@Ons-joe, try any of the following CSS codes:

/* Hide Tab Bar contents (Doesn't remove title bar)
#TabsToolbar .tabbrowser-tab,
#TabsToolbar .tabs-newtab-button,
#new-tab-button, #alltabs-button {
display: none !important;
} */

/* Hide Tab Bar contents (Removes title bar) */
#titlebar {
visibility: collapse !important;
}

/* The margin-top so that the buttons and search bars on the nav bar aren't cut-off when Firefox is maximized */
#main-window[sizemode="maximized"] #nav-bar {
margin-top: 8px;
}

/* To hide the sidebar header */
#sidebar-header {
display: none !important;
}