Mr Uptime a Firefox add-on which will let you know about changes in websites which usually have problems or are unavailable.
Mr Uptime check continuously the availability of hose websites you want it to check and it will warn you when changes take place. By that way, you will not have to check it manually, because Mr Uptime will tell you when it's up again.
Mr Uptime can be really useful, because it can be also used to know when changes take place and not only to know when they are ok. If you are usually surfing the net, Mr Uptime can be a good friend.