LeechBlock isn't some over-restrictive "nanny" that automatically blocks everything remotely interesting. Instead, it gives you options to decide what to block and when. When you open LeechBlock's options from Firefox's Tools menu, it gives you a tabbed window in which you can make up to six renameable "block sets." Each of the block sets can have its own rules for the days of the week and time of day for blocking. You can allow each set a certain number of minutes before it cuts you off. By default, LeechBlock redirects you to its own page when a more compelling one is blocked, but you can choose a different one if you prefer.
This customizability makes LeechBlock a great productivity tool for folks who need just a little Web control, not total self-denial.
Note: This link takes you to the vendor's site, where you can automatically install the file into your Firefox browser.