Does it work when you turn off HTTPS Scanning in Burp? This isn't a useful security feature, in fact it makes matters worse.

Also see https://support.mozilla.org/en-US/kb/error-codes-secure-websites

Thanks Christ1 for your reply.

I am just trying to find out how to turn it off in Burp and will post back. I am new to both Firefox and Burp. This is what I see at the moment (attached) in Firefox. Thanks

Are you running a local proxy on your machine? If so, why?

Or was that part of the Burp instructions? If so, revert to the original setting.

Thanks for our patience.

I have managed to resolve the certificate problem, but I am getting a Firefox error whether I am trying to simply view a http page (any page) or https page (any page).

The proxy server is refusing connections

Firefox is configured to use a proxy server that is refusing connections.

Check the proxy settings to make sure that they are correct. Contact your network administrator to make sure the proxy server is working.

Again, this is what I currently have (attached screenshot).

I have ticked the Socks6 box but get the same error, and the DNS HTTPS box - same error - and then selected both at the same time and, again, the same error.

I have restarted the PC (Win 10), but it has made no difference.

Strange thing is that it worked yesterday for both http and https sites, but just not today.

Thanks again.

Unless you're running a local proxy (for whatever reason), simply set it to 'No proxy'.

Thanks again.

Yes, if I set it to 'No proxy' and visit http://localhost/ a per the Burp tutorial, I see this:

Unable to connect

Firefox can't establish a connection to the server at localhost.

   The site could be temporarily unavailable or too busy. Try again in a few moments.
   If you are unable to load any pages, check your computer's network connection.
   If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the Web.

If I go to https://www.mozilla.org/en-GB/ I can see the Web page.

I probably can't access http://localhost/ while the browser is set to 'No proxy'.

I will go over the tutorial again, but I am sure I followed it correctly.