XSS Me is a Firefox plugin that helps you secure your forms and user input areas by performing a list of well-known XSS attacks, then reporting back with test results. It doesn't actually harm your site, it just detects whether your input areas are sensitive for XSS-like attacks.
Link:
https://addons.mozilla.org/en-US/firefox/addon/7598

Please note: if you're passing all the tests from this plugin doesn't mean your website is 100% secure, it's just a tool to help you.