Firefox can't find the server at......error

First, boot to Safe Mode with Networking. Does it work from there. If yes, boot normally then disable Eset and see if it works. If yes then you need to configure Eset to allow you to get out to the internet. No idea how you do this as I've never used Eset.

Try using Internet Explorer, see if the same problem occurs. If not then it's Firefox specific and we'll know where to focus. Is it a specific web sites or any website? As a test, uninstall Eset and see if the problem still occurs. If no, switch to a different Av/Firewall program or figure out how to allow Firefox through the Eset firewall.

Try using Internet Explorer, see if the same problem occurs. If not then it's Firefox specific and we'll know where to focus.

Is it a specific web sites or any website?

As a test, uninstall Eset and see if the problem still occurs. If no, switch to a different Av/Firewall program or figure out how to allow Firefox through the Eset firewall.

The problem has also occurred in I.E also & is not website specific.

I have also spoken to the good people at ESET & established the problem is not Firewall related.

Time to talk to my ISP?

Is this your XP or Vista system?

Have you tried running Firefox without Add-ons? I believe you can do this by clicking on the FIrefox button at top left, then Help then Restart Firefox with Add-ons disabled.

If no problem then one of the Add-ons is the culprit, though this seems unlikely since you say it also happens with IE. If, for some strange reason it does make a difference then you can use Firefox / Add-ons to find the one that is causing trouble.

You can also do likewise for IE, by starting it with no Add-ons via Start, Accessories, System Tools, Internet Explorer (No Add-ons).

Edit: Even though ESET says it's not a firewall problem, I still think it would be worth while uninstalling ESET and testing to make sure. Perhaps install Microsoft Security Essentials while ESET is uninstalled for safety?

Also check to see if a hosts file is being used: C:\windows\system32\drivers\etc

Depends, what are the entries? Easiest way to see if it will make a difference is rename hosts to hosts.save then reboot and see how things work.

Sorry I don't quite understand the thinking behind renaming the host file.
I followed this Microsoft KB article HERE to reset the host file.
As a cosequnce of this I now have two host files; one named 'host' & 'host.old' respectively.
Will this cause a problem? Can the .old host file be deleted?

Renaming the hosts file has the same effect as what you did so all is well. Yes, you can delete the hosts.old. Doing this did not make any difference, right?

Okay, so disabling ESET, getting rid of the hosts file and running Firefox in Safe mode makes no difference and the problem also exists with IE.

Is this on your XP system? If so, give WinsockXPFix a try, see if it helps.