This is a Firefox Add-on port of defunkt's Chrome extension https://github.com/defunkt/dotjs
If you navigate to http://www.google.com/
, dotjs will execute ~/.js/google.com.js and/or ~/.js/google.com.coffee. If you have a ~/.js/default.js, it will execute on every page you visit. Also, you can put site specific .css files in ~/.css (C:\Users\<username>\css\. in Windows 7). default.css loads in all sites.
Bonus: files in ~/.js have jQuery 1.9.0 loaded, regardless of whether the site you're hacking uses jQuery.
GreaseMonkey user scripts are great, but you need to publish them somewhere and re-publish after making modifications. With dotjs, just add or edit files in ~/.js.