Hi, please see this https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/ Answer to question is here : https://support.mozilla.org/en-US/questions/1185016 as well type new tab into the search box in this URL something may interest you : https://addons.mozilla.org/en-US/firefox/extensions/ fyi : USSER CHROME URL'S *https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/ *http://techdows.com/2017/09/classic-theme-restorer-userchrome-css-modify-firefox-57-photon-ui.html *https://github.com/Aris-t2/CustomCSSforFx/issues/1 *https://www.accessfirefox.org/Theme-Font-Size-Changer.php *https://www.userchrome.org/ *https://www.accessfirefox.org/Firefox_Accessibility_Themes.php *https://www.userchrome.org/what-is-userchrome-css.html *http://kb.mozillazine.org/UserChrome.css *https://github.com/axydavid/FirefoxUI/blob/master/README.md *https://github.com/wilfredwee/photon-australis *https://www.reddit.com/r/FirefoxCSS/ Note not all code in one place. Please let us know if this solved your issue or if need further assistance.