You may use the HTML and CSS files on www.mozilla.org or other mozilla.org websites for your web site - as long as you don't use the visual design or style (the colors, gradients, fonts, etc.). In other words, you can use the HTML and/or CSS code, but it should not look like you took our page and dropped in your logo.
mozilla.org does not intend to exercise any copyright it may have in any HTML/CSS underlying its pages to prevent you from copying or reusing that HTML/CSS. However, this does not grant you any right to use the trademarks, trade names, service marks, or trade dress of Mozilla Foundation. For information on our trademark policy, see the Trademark and Logo Usage Policies.