How to Troubleshoot
There are a couple of things you need to do in order to find out why you are getting this error. Here are some suggestions on how to troubleshoot this problem:
1. Check your Internet connection. It is unlikely that your Internet connection is down, but you must be sure it is up and running. If there is at least one program which can access the net, then you know that your Internet connection is fine.
2. Make sure that Firefox can access the net. After you have made sure that your Internet connection is fine, you need to make sure that Firefox can access the web. Try opening any other site in Firefox and, if it works, this means that Firefox is not blocked by a firewall or another piece of software. If you can't open a site in Firefox, the most likely reason is that Firefox is blocked by a firewall or another application. In this case you need to find the firewall/application that is blocking Firefox and set the browser free.
3. After you make sure that Firefox can access the Web, try again loading the problematic page. If the message appears again, try to load the same page in a different browser (i.e. Internet Explorer). If the page loads, it means that something is preventing Firefox from loading it. If the page can't be loaded in Internet Explorer either, but you know that the site is not down (you can ask a friend to load the same page from a different location). It is possible that a firewall your network admin has enabled is preventing you from accessing this particular page or the site as a whole. You could discuss the issue with your network admin and see if there is something he or she can do.
4. Check for firewalls and other applications locally on your computer which might be blocking Firefox access to the particular page. A local application may be blocking Firefox. This application could be a firewall, spyware, adware, or an anti-malware program. You need to check the settings of your firewall(s) and anti-malware programs and if you find any lists of blocked URLs/IPs, look for the URL/IP of the page/site you want and remove it from the blocked list. The process of detecting what is blocking Firefox from accessing a particular page might be gruesome because the reason is not always obvious – sometimes anti-malware programs just block a site they deem suspicious and you are left in the dark why this site is not accessible for you.