To block a site without any software or FireFox filters. just follow my guideline
Go to C: Windows--> System32 --> driversetc.
The "hosts" file with no extension can be found.
Open it with the Notepad.
Getting Started with the bottom of the page. And search the line "127.0.0.1 localhost".
Add next line next to that line "127.0.0.1 http://www.xyz.com/localhost". Where xyz is the site you want to block.
Now open the site in Internet browser and check it.