LeechBlock is a nice productivity tool for your FireFox Browser. In fact it is like your own personal Web Sense / Firewall software. You can block the sites, which wastes your time during your work day!. You can also specify the blocking duration. Even the options pages of this extension can be password protected, to slow you down on a tempting browsing mood.

You may install the LeechBlock extension to see if works for you. We have added a few forums to LeechBlock , which  we cannot resist otherwise.