1. Open Firefox and click on the "Tools" menu. Select "Add-ons" and then click "Get Add-ons."
2. Type "BlockSite" into the search bar and hit "Enter." Click on "Add to Firefox," "Accept and Install," and then "Install Now." If you get a message saying that Firefox needs to restart, click on the "Restart Firefox" tab, or close your browser window and then launch the browser again.
3. Go back to the "Tools" menu once your Firefox browser restarts. Then click on "Add-ons" and find BlockSite in your list of Add-ons. Click on the "Options" button.
4. Make sure the boxes "Enable BlockSite," "Enable warning messages," and "Enable link removal" are all checked under the "Enable functions" heading. Click the radio button next to "Blacklist."
5. Click the add button, type or copy the URL of the website you would like to block in the text box, and click "OK."
6. Put a checkbox in the "Enable authentication" box and type in a password if you would like the use of the BlockSite add-on to be password protected. This will make it so users will need to enter the password in order to add, edit or remove websites from the list of blocked websites.
You can use the import and export feature if you wish to copy the list onto other computers that have the BlockSite add-on for Firefox. Click on the "Export" button, type in a file name, and click the "Save" button. Store that saved file onto an external drive, such as a USB flash drive, and transfer it onto the other computer. Then launch Firefox, open the BlockSite add-on, and click the "Import" button. Choose the file you saved from the other computer and click "OK."
You can also use BlockSite to create a list of websites that you want users to be able to view by using the Whitelist feature. When you are adding websites to the list, make sure to check the radio button next to "Whitelist" rather than the button next to "Blacklist." Users will then not be able to view websites that are not on the Whitelist.
If you are using Windows and you forget your BlockSite password, you can recover typing "about:config" in your browser address bar and pressing Enter. You can either right-click on "BlockSite.authenticate" and choose "Reset," or delete the "BlockSite.password" entry. This will reset everything so you can access the BlockSite add-on settings without entering the password.
http://www.wikihow.com/Block-Websites-on-Firefox