これは古いやり方です。次のWikiを参照してください。 http://wiki.nothing.sh/page?userChrome.js%CD%D1%A5%B9%A5%AF%A5%EA%A5%D7%A5%C8#m5c944e2

mozilla.cfg

// -*- mode: javascript;-*-
/**
 * @file 「mozilla.cfg」(autoconfig.cfg) として動作するよう修正した簡易版userChromeJS。
 *       The simplified version of userChromeJS which modified to run as “mozilla.cfg” (autoconfig.cfg).
 * @version 3.0.2
 * @see [Deploying Firefox in an enterprise environment — Mozilla | MDN]{@link https://developer.mozilla.org/Firefox/Enterprise_deployment#Configuration}
 * @see [mozdev.org — userchromejs: index]{@link http://userchromejs.mozdev.org/}
 * @see [[Ext] userChromeJS 2.0 [2015-08-02] • mozillaZine Forums]{@link http://forums.mozillazine.org/viewtopic.php?p=14747866#p14747866}
 * @see [設定の管理 | 技術的なよくある質問 | 法人向け情報 | Mozilla Japan コミュニティポータル]{@link https://www.mozilla.jp/business/faq/tech/setting-management/#mcd}
 * @see [userChrome.js/userChrome.js at master · alice0775/userChrome.js]{@link https://github.com/alice0775/userChrome.js/blob/master/userChrome.js}
 * @license
 * ***** BEGIN LICENSE BLOCK *****
 * Version: MPL 1.1/GPL 2.0/LGPL 2.1
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the 'License'); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 * http://www.mozilla.org/MPL/
 *
 * Software distributed under the License is distributed on an 'AS IS' basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is the userChrome.js component.
 *
 * The Initial Developer of the Original Code is
 * Simon Bünzli <[email protected]>
 *
 * Portions created by the Initial Developer are Copyright (C) 2007
 * the Initial Developer. All Rights Reserved.
 *
 * Contributor(s):
 * alta88 <[email protected]>
 *
 * Alternatively, the contents of this file may be used under the terms of
 * either the GNU General Public License Version 2 or later (the 'GPL'), or
 * the GNU Lesser General Public License Version 2.1 or later (the 'LGPL'),
 * in which case the provisions of the GPL or the LGPL are applicable instead
 * of those above. If you wish to allow use of your version of this file only
 * under the terms of either the GPL or the LGPL, and not to allow others to
 * use your version of this file under the terms of the MPL, indicate your
 * decision by deleting the provisions above and replace them with the notice
 * and other provisions required by the GPL or the LGPL. If you do not delete
 * the provisions above, a recipient may use your version of this file under
 * the terms of any one of the MPL, the GPL or the LGPL.
 *
 * ***** END LICENSE BLOCK *****
 * @author 100の人
 * @see {@link https://gist.github.com/esperecyan/5b9f957abf64749cc9529e130b655d64}
 */

try {
	Components.utils.import('resource://gre/modules/Services.jsm');

	new class {
		constructor()
		{
			Services.obs.addObserver(this, 'profile-after-change', false);
		}

		observe(aSubject, aTopic, aData)
		{
			switch (aTopic) {
				case 'profile-after-change':
					Services.obs.addObserver(this, 'final-ui-startup', false);
					break;

				case 'final-ui-startup': {
					const file = Services.dirsvc.get('UChrm', Components.interfaces.nsIFile);
					file.append('userChrome.js');

					if (!Services.appinfo.inSafeMode) {
						this.mFileURL = Services.io.getProtocolHandler('file')
							.QueryInterface(Components.interfaces.nsIFileProtocolHandler)
							.getURLSpecFromFile(file);

						Services.obs.addObserver(this, 'domwindowopened', false);
					}
					break;
				}
				case 'domwindowopened':
					aSubject.addEventListener('load', this, true);
					break;
			}
		}

		handleEvent(aEvent)
		{
			const document = aEvent.originalTarget;
			if (document.location && document.location.protocol === 'chrome:') {
				try {
					Services.scriptloader.loadSubScript(this.mFileURL, document.defaultView, 'UTF-8');
				} catch (ex) {
					Components.utils.reportError(ex);
				}
			}
		}
	}();
} catch (exception) {
	displayError(exception);
}