If people can connect to a domain, they should also able to block it. This should be a basic funcation on OS and browsers. Also LeechBlock do not prevent the website connection. If you need to block it , you need a adblocker such as uBlock Origin.
Copy/paste the above code (example) into Notepad Save Type: All files policies.json
Then Create the distribution folder Move policies.json to the distribution folder Move the distribution folder to the folder where the Firefox exe file is located
Use the Troubleshooting Information page to help fix Firefox issues Enterprise Policies: This shows you if your browser is being managed by Windows Group Policy or a policies.json file. If enterprise policies are managing Firefox, clicking on Active will show you a list of the policies that are currently active on Firefox.