How to block www.facebook.com at Firefox

I take it you have Windows OS?
This is a simple fix for your question:

Open My Computer
Open C:
Open Windows
Open System32
Open Drivers
Open ETC
Open the hosts file notepad or wordpad

Add this right at the bottom:
0.0.0.0 Welcome to Facebook (http://www.facebook.com)
0.0.0.0 Welcome to Facebook (http://www.facebook.co.za) "<- Depending which country you are from, the ZA will change to UK or AU...

Restart you computer and done.

Hope this helps...

Cheers