Mr Uptime is a Firefox extension that will keep trying to reach a website that gone down. It will keep trying in the background while you continue surfing on other websites. Mr Uptime will alert you, when the problematic website is accessible again.

Mr Uptime can be used for any kind of website error such as temporary maintenance pages, 404 and other HTTP server errors, server-side scripting errors, pages where not all elements load correctly, database errors, and too many server connections.

With Mr Uptime, you can set aside downed sites, and return to them later when they are up. All you need to do is add the URL to your Mr Uptime watchlist and then continue surfing other websites. You will never miss a website again. Mr Uptime will monitor the downed site every 5 minutes for the first hour, every 10 minutes for the next 120 minutes, every 15 minutes for the following 160 minutes, and every 30 minutes after that. You can configure it to open in a new tab or window on your browser,give you a small alert, clear watchlist when exit Firefox and set monitoring time.

Features:

• Status change
  Watch when the server status code changes from an error to OK (HTTP status code 200).
• Keywords
  Set a keyword that the website should or should not contain to be considered ok.
• Time to watch
  Set how long you want to keep checking a website. After this time it will be removed from the watchlist even if it hasn’t become available.
• Notifications
  Choose to open the website in a new tab or a new window when it becomes available. You can also optionally get an alert dialog box to notify you when a website is working again.
• Easy to use and discreet
  The interface will not clutter your Firefox window. The toolbar is only shown when an error is encountered or when you choose to open it yourself. You will not get another toolbar taking up valuable space in Firefox.

Mr Uptime is free, you can download it from the Mozilla Addons site or from Pingdom.