As discussed in the previous post Blocking Websites or how to blockURL is a common problems mainly faced by parents and employers.

Many parents worries about children wasting time on the Internet by chatting and browsing through inappropriate contents.Children could be interacting with complete strangers and even sharing personal information and this should be controlled to avoid unwanted consequences.
We have discussed about How to Block URL or Web Site in Internet explorer in the last post.
Read: How to Block URL or Website in Internet explorer?

Option available in Firefox browser to Block Websites

FoxFilter Add-on for Firefox

FoxFlter is known as the Parental control for Firefox.FoxFilteris a personal content filter that helps block pornographic and other inappropriate content.The free version supports web filtering and this is enough for blocking website and their URLs.But if you want to protect these configurations  by adding a password then you need to register to avail Premium features. Premium features are available with small support fee.

DOWNLOAD and INSTALL FoxFilter Add-on from mozilla.org.
After installation go to Tools -> FoxFilter Settings
A new web page will open there you can set the keywords in the Settings menu as below.The keyword should be the keywords of website URL that you want to block.For example if you want to block Facebook add “facebook” in the keyword list.Also under the sensitivity settings you can set the sensitivity of the keyword.  Bottom line, the most sensitive approach is to examine the Body content where it will check for the presence of keyword in the body content Block URL in Firefox - FoxFilter