LeechBlock is a simple productivity tool designed to block those time-wasting sites that can suck the life out of your working day. You can specify up to six sets of sites to block, with different times and days for each set. You can block sites within fixed time periods (e.g., between 9am and 5pm), after a time limit. You can also set a password for access to the extension options, just to slow you down in moments of weakness.
What is new in this release:
Version 0.6.3 added quarter-hour and half-hour options for time limit period, support for Firefox Sync, and reduced number of preferences.
Mozilla Firefox 4.0 or later