Many site issues can be caused by corrupt cookies or cache. In order to try to fix these problems, the first step is to clear both cookies and the cache. Note: This will temporarily log you out of all sites you're logged in to. To clear cache and cookies do the following:

1. Click the menu button , choose History, and then "Clear Recent History...".
2. Under "Time range to clear", select "Everything".
3. Now, click the arrow next to Details to toggle the Details list active.
4. From the details list, check Cache and Cookies and uncheck everything else.
5. Now click the "Clear Now" button.

Further information can be found in the Delete browsing, search and download history on Firefox article.

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Thanks for your response, I always clear cookies and cache after each session anyway, so I don't think this would be the issue. So far it seems to be intermittant as well, for example, last night it did not work for about half an hour and then I tried it again a little later and it worked OK.

It was not working half an hour ago, which led to my posting this enquiry, but I just tried it and it is working fine again...

It just happened again, when I first open the browser it comes up with a 'Server Not Found' error. I try direct from the home page button and a refresh and still 'Server Not Found'. I then left it 5-10 minutes, pressed the home page button again and it opens just fine.

If anyone can suggest why this is suddenly happening or how I can fix it I'd appreciate it.

Seems to have solved itself whatever it was, thanks for your responses though.