nixpkgs/home/programs/firefox/kioskLikeUserChrome.css

/*
 * Important: use "visibility: collapse" instead of "display: none".
 * I do not know why, but display: none will hide the whole content of the frame.
 *
 * See: https://firefox-source-docs.mozilla.org/devtools-user/browser_toolbox/index.html
 */
#navigator-toolbox {
    visibility: collapse !important;
}