I would recommend doing this at the windows level rather than in firefox since then the user cannot get around the block by using a different browser. See here: [[Parental controls]] However if you want to just block sites on firefox and leave other browsers unaffected you can use an add-on such as BlockSi. [https://addons.mozilla.org/en-US/firefox/addon/blocksi] That should allow you to set up a custom list. Let me know if that doesn't work.

Actually I think this addon might be even better suited: https://addons.mozilla.org/en-US/firefox/addon/leechblock/