See screen shots: https://addons.mozilla.org/en-US/firefox/addon/6562/
The trouble about doing it for one browser is that your other browsers quickly come into use.
One system wide fix is to edit your hosts file: http://thingsilearned.com/2007/05/04/combating-internet-addi...
but it only blocks you from sites instead of giving you nice time limits. And it becomes too easy to change back.
I wish there was something that offered the limit functionality but for all browsers/access but currently i know of nothing.
RescueTime (YC W08)'s Get Focused app is the best implementation of this I've seen.
1) The icon needs some work. Not sure what you're trying to represent.
2) When you click enable/disable in the popup menu, the menu should automatically disappear. You shouldn't have to click again to dismiss it.
3) If while you have the extension disabled you load a page in your blocked list, then enable the extension, then refresh the page, the page doesn't get blocked.
The refresh thing has to do with the way chrome handles this event. I'm working on a work around for it right now.