If you don't trust yourself enough not to waste time visiting games, video and social networking sites when you should be working, then LeechBlock could be what you need.
Alternatively, LeechBlock could be used to stop you being constantly plagued by ads or messages from sites that you have no interest in. It could even be used by concerned parents to block kids from visiting certain sites in Firefox (although the developer does not advise using it for this purpose).
The nice thing about LeechBlock is that you can configure it to block sites between a specific time period, so you don't have to keep activating and de-activating it if you want occasional access to a site. This includes between certain times, specific moments in time, or a combination of both.
Principally though it's been designed for those with a wandering mouse cursor, which is why it even allows you to set a password to access prohibited sites to help "slow you down" in moments of weakness. If you can't help yourself, then LeechBlock will track the amount of time you've spent on blocked sites. Adding sites to your 'blocked' list is easy - just go the the LeechBlock extension in your add-on manager and click 'Options' to add a site.
The concept of LeechBlock is admirable enough, although the fact that you set the password for accessing sites means it's not really going to stop you if you get tempted. I'd be more likely to use it as a way of blocking annoying sites or other people accessing sites on my machine.
If you really can't trust yourself not to keep checking your email every two minutes or get distracted playing poker when you should be working, LeechBlock can help you break the habit.