Could you check these articles:

• What do the security warning codes mean?
• How to troubleshoot security error codes on secure websites

If you are getting the other error page, see:

• Secure connection failed and Firefox did not connect

When attempting to load any (& more) of the sites listed I get the following:

Your connection is not secure The owner of www.tenforums.com has configured their web site improperly. To protect your information from being stolen, Firefox has not connected to this web site.

Seriously, are all of the sites I listed improperly configured: hard to believe.

That message is most likely wrong. Most likely there is a "man in the middle" of your browsing connection that is causing this problem on all of the sites. Could you check the articles I linked to before for suggestions on how to investigate that. Hopefully it's your security software and not some malware.

In one of the articles it says I should turn off Scan SSL in my AV. I'm using Bitdefender Internet Security 2019 & cannot find any reference to Scan SSL. However, using the same AV with Chrome there are no problems.

However, there is a setting for 'Encrypted web scan' which when turned off appears to solve the problem. I feel uncomfortable doing this since, as I said, it's only with Firefox, not Chrome where this happens. But it works so thanks for your help.

Your security software often can set up Firefox to trust its fake certificates. This may be something it checks at system startup, or there may be a panel in the security software's settings to push its signing certificate into Firefox. Or there might be instructions on the vendor's site or forum, although with Bitdefender, the forum posts weren't very helpful the last time I checked (probably 18 months ago now).

Since I turned off the offending setting in the AV Bitdefender complains loudly that I'm at significant risk. So, I'll contact Bitdefender & see what they have to say & report back. However, it makes it understandably uncomfortable using Firefox.