How to Block Websites in Mozilla Firefox? Open Firefox Browser Click “Tools” then “Add-ons.” Click “Get Add-ons” tab. Type “Block site” in the search bar, and hit “Enter”. Click the “Install Now” button next to block site . Restart Firefox when prompted. Click “options” button. Make sure the “Blacklist” radio button is ticked, as well as “Enable Block Site,” “Enable warning messages” and “Enable link removal.” Check “Enable authentication,” then enter a password if you would like to keep other users from deleting blocked websites or changing the settings in Block Site without your permission. Click the “Add” button, enter the URL of the website you would like to block, then click “OK.” Once the list of blocked sites is complete, click “OK” and restart Fire fox. As you can see the website is now blocked in Mozilla Firefox.