"4.6 You may uninstall the FBDownloader Software Application at any time. Each component of the application should be uninstalled separately from the control panel and from the browser(s) that you use".

at http://fbdownloader.com/lp/privacy/eula.html , but I have no illusions that it will work.

So, open your browser, (you don't say which browser), and find Manage Add-ons and disable this add-on. Then head over to the Control Panel and try to find whatever this is listed as in "Programs and Features" and uninstall it from there.