I could have rate it 5 stars if it had the ability to either: configure 1) different schedules for different sites in the block list; or 2) at least configure different schedules for different days of the week, even if for all sites in the block list (as a whole). Examples for both scenarios:
1) block site A for Mo/Tu/We/Th/Fr from 19:00 to 00:00 and for Sa/Su from 10:00 to 00:00;
block site B, period!
2) block all the sites in the block list for Mo/Tu/We/Th/Fr from 19:00 to 00:00 and for Sa/Su from 10:00 to 00:00;

Another useful feature would be to allow a given time interval (in timer or countdown style) to access a site in the block list, if defined by the user as an extra to the scheduling, again either 1) site by site; or 2) as a whole, that is, applied to all sites in the block list. Example: I want to have the possibility to access Site A for 01:00 any day of the week, regardless of the scheduling rules defined above. That 01:00 time frame would reset every midnight.

Nevertheless, it's a good Firefox extension! ;-)