Extensions are meant to enhance your browser's abilities. Some, like adblockplus or noscripts may be unintentionally blocking certain icons on your site.
To test, turn off suspected extensions and do a hard reload of your page. (Unlike a standard page-refresh, a hard reload of your site forces the browser to re-download every javascript file, image, etc. You can initiate a hard reload by either emptying the cache first and then reloading the page normally or by a direct hard-reload via your browser's dev tools).
ref: https://github.com/FortAwesome/Font-Awesome/wiki/Troubleshooting