Whether you want to protect your children from visiting suspicious websites or you want to deny access to websites for any other reason, BlockSite extension for Firefox is the right choice for you. It 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.
Open your Firefox browser.
From the horizontal main menu of Firefox, select Tools>Add-ons (if you do not see the menu, press the Alt key on your keyboard to appear.)
Go to Get Add-ons on the left side of Firefox.
On the top-right corner type “blocksite” in the search bar and hit “Enter.”
Look for the BlockSite add-on and click the install button.
After installation completes you will be prompted to restart Firefox.
After Firefox restarts, go to Tools>Add-ons, and select the extensions tab on the left side of Firefox.
Look for BlockSite extensions you’ve just install and click on the Options button.
After the pop-up window appears, make sure the “Blacklist” button is ticked, as well as “Enable BlockSite,” “Enable warning messages” and “Enable link removal.”
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.
If you would like to keep other users from deleting blocked websites or changing the settings in BlockSite without your permission, check Enable authentication and enter a password.