Put a few numbers into the "surfHours" array and you'll be able to access your "blocked" sites for one hour after each hour listed. As an example, if surfHours = [9,13,17] you can access facebook (or any other site) from 9-10am, 1-2pm and 5-6pm.
You can install the script directly from userScript and the source code is below. If you don't know how to install a grease monkey script, check out the instructions at http://userscripts.org/scripts/show/92640
surfHours tstamp hours tstamp okToView x x surfHours length x hours surfHours x okToView okToView b document b