This site is usually safe. It's a userscript site - userscripts are like browser extensions, but programmed in Javascript. That said, as with any browser extension you should be cautious. You can read the source code because it is Javascript so I recommend you do so first. Don't accept any userscripts for Facebook. They're all scams of course. But usually a safe site.

This website provides scripts mainly aimed for the Firefox add-on Greasemonkey. Many scripts will also run natively in Chrome and Opera. A great amount of scripts is also compatible with the following extensions: Scriptish (Firefox), Tampermonkey (Chrome), NinjaKit (Safari & Chrome). Userscripts are tiny bits of JavaScript using which the way websites look and work in browsers can be modified. WARNING Please be warned that, from time to time, there will appear fake scripts that claim to "Hack Facebook" or other accounts / sites. Please be advised that any script which has a lot of positive reviews from accounts that also reviewed similar scripts, little discussions and was updated recently is almost certainly fake, ESPECIALLY if such scripts ask you to download anything from another site!

a good user script for facebook. FFixer Script Summary: Enhancements for Facebook: bigger profile pictures and photos, easier viewing of albums, links to download videos, showing people's age and sign, google calendar integration, bookmarks, keyboard shortcuts & more. Fully customizable!