Hello again
Thank you to both of you for the additional contributions. Nothing suggested has led to a resolution in my case.
I have gone through Icecat and Firefox about:config looking for differences. There are a few but none that seemed to be significant with regard to the issue I have, or that I can fully understand their implication yet lets say (other than a good guess).
I have found one that was different and I matched Icecat to Firefox and this allowed websites that were not displaying to be displayed without the error page I posted.
That parameter/setting was: security.ssl.require_safe_negotiation
Setting it to false as per Firefox's default, allowed the page to be displayed, but with lots of warnings and the ability to see what was wrong (insecure and weak keys etc being reported in my case).