A simple webextension that allows you to block those instant gratification websites that keep on stealing your valuable time.
(LeechBlock is similar, but much more configurable)
Download and install now from the Firefox Add-On repository
A grey pie should show up in your menubar. Navigate to the offending site, browse it, then, once you're thinking that you really have to get back to work/study/vacuuming, click on the icon, enter the time you want it to be blocked and hit
Enter. You simply will not be able to see that site again until the block expires.