Is there a firefox plug-in that gives the option to limit time on certain sites or pops up to tell me times passing instead of all block?

This would probably be useful for helping people manage their productivity – something that will give you a little nudge to get off a social networking site & get back to work. There’s nothing like a ticking clock to guilt you into getting away from Twitter or Facebook and back into whatever work you’ve been avoiding!

Taken from the TwitEye Ideas Stream.