How to Block Websites in Mozilla Firefox

The easiest way to block websites in Mozilla Firefox is by using a free, third-party add-on called BlockSite. The BlockSite 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. BlockSite also has a "password protection" feature that helps prevent unwanted changes to BlockSite by others.
Instructions

1. Open Firefox, click "Tools" then "Add-ons." Click "Get Add-ons" and then "Browse All Add-ons." Type "blocksite" in the search bar after the Mozilla Firefox add-ons website opens and hit "Enter."

2 .Click the "Add to Firefox" button next to the entry for BlockSite. Allow the add-on to load (usually fewer than 5 seconds), then click the "Install Now" button. Restart Firefox when prompted.

3. Go to "Tools," then "Add-ons," then click the BlockSite "Preferences" button after Firefox restarts. Make sure the "Blacklist" radio button is ticked, as well as "Enable BlockSite," "Enable warning messages" and "Enable link removal."

4. Check "Enable authentication," then enter a password if you would like to keep other users from deleting blocked websites or changing the settings in BlockSite without your permission.

5. Click the "Add" button, enter the URL of the website you would like to block, then click "OK." Repeat to add more sites, or click the "Clear" button to delete the list. Once the list of blocked sites is complete, click "OK" and restart Firefox.

Tips & Warnings

To import a previously prepared or exported list of blocked websites, click the "Import" button in the preferences window, browse to the file you would like to import and click "OK." The file should have "[Block Site]" listed at the top of the page followed by the URLs of the sites you would like to block.