It would make more sense to block from the actual router than creating individual rules in each browser. There are also issues with employees being able to get around it, so maybe you'd want to get a free antivirus program like Avast.. password protect it.. and place the settings in there (if no router is being used).

The easiest way to block websites in Mozilla Firefox is by using a third-party add-on called Web Filter. The Filter extension blocks sites that have been added to its "blacklist," but it can also be set up to disable any links to the blocked sites by showing nonfunctional text instead of hyperlinks. The Filter also has a "password protection" feature that helps prevent unwanted changes to BlockSite by others.