This is an extension I mentioned here before.  It will keep trying to reach the web site in the background while you continue to surf to other sites.  As soon as the web site in question is back, Mr Uptime will give you a tap on the shoulder to let you know.  Here are a few more words on its 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 watch list 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.

You can learn more about the Mr Uptime add-on via the Firefox Add-ons page for it or visit the official site at mruptime.pingdom.com.

This is the first time I can remember seeing all the Mozilla web sites down, wonder if they are fixing a technical problem, or maybe getting ready to launch something new?  Let the speculation begin!