~/Library/Application Support/Firefox/Profiles/xxxxxx.default/chrome/userChrome.css

userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* to hide the native tabs */
/* #TabsToolbar {
    visibility: collapse;
} */
/* to hide the sidebar header */
#sidebar-header {
    visibility: collapse;
}

#tabbrowser-tabs {
    visibility: hidden !important;
}

/* hide the bookmark icon */
toolbarbutton.bookmark-item > .toolbarbutton-icon {
  display: none;
}