Block a URL at browser level
The way I would do it is to install greasemonkey (if you don't already have it)
Then you could write your own script (right click the icon, add new user script). When it asks you where to apply it to, enter "http://*superuser.com/*"
The script contains:
window.location.href = "about:blank";
During your lunch-break you simply de-activate that script from greasemonkey
2 firefox extensions - leechblock and blocksite
Use AdBlock Plus to block URLs from the browser