How do I block access to specific websites using Firefox?

There used to be a good add-on that allowed me block websites but now it’s lapsed. I tried blocking websites by altering the hosts file but it doesn’t seem to be working and I can’t figure out what is going on.

I followed this advice to block the sites and the only issue that arose was my anti-virus popped up saying “In accordance with security guidelines, the Administrator has blocked access to the Hosts file” and I’m trying to figure out if Avira blocked access to the hosts file or if the stupid (but seemingly effective) software is not blocking access to the hosts file. If so how to I get the antivirus software to unblock the hosts file? (I’ve checked online and not found anything helpful thus far).

Can the techies of the group help me?