Thanks but better use a style for this purpose Rated 3 out of 5 stars

After using this add-on for a long time, I've found a style doing the same work : "Firefox Orange Menu Button Hider" (http://userstyles.org/styles/43377/firefox-orange-menu-button-hider). Thanks.

P.S: You will need the add-on "Stylish" for installing the style ; https://addons.mozilla.org/en-US/firefox/addon/stylish.