Limiting USER accounts from using Mozilla to surf the web

I have always used Mozilla at home, but I have not used it at my work because we do not allow employees to surf the web. Its easy to block for Internet Explore, but I am at a loss how you can block this with Mozilla. Do you know of an add-on that will not allow my employees to look at websites? Or is there an option already built into Mozilla.

Thank you.

Hello,

Take a look here:

http://www.howtonetworking.com/Internet/restrictie0.htm

And here: http://www.wikihow.com/Restrict-Web-Browsing-Using-Firefox

Thank you for the help.