The only way to modify ui is by adding custom CSS code to userChrome.css and userContent.css files inside the browser's profile folder. The only way to modify UI is by adding custom CSS code to userChrome.css and userContent.css files inside the browser's profile folder. Keep in mind that CSS code can not create entirely new items, buttons, or toolbars. It only can modify already present UI items. Keep in mind CSS code can not create entirely new items, buttons, or toolbars. It only can...