To disable the featureloader stuff from JomSocial, in the admin area, go to JomSocial. Then click "Configuration" and "Facebook Connect". In there, remove your API and Secret keys and set all options to 'no'.
Opening the HTML on the page depends on your browser. In most, you usually want to go to the "Tools" (or similar) menu and use the "View Source" option. That will show you all the 'code' that generates what's on your page, and shows all the inclusions of different files and/or Javascript. Again, once you load that, just search for "Featureloader.js.php", and you should see the section of code that you're trying to remove. Hopefully, with the information above about JomSocial will do it for you.
Thanks,
Alex