You might be able to find out if you are having some issue with DU's security certificate by opening the Add Security Exception dialog by pasting the following in FireFox's address bar:

chrome://pippki/content/exceptionDialog.xul

Then paste https://www.democraticunderground.com/ into the Server Location field and click the Get Certificate button (that may resize your browser window - the dialog is actually supposed to be a popup, not a normal window).

Hopefully, the resulting Certificate Status information will reveal the error. If there is some problem, you could add a Security Exception to see if you could then connect to DU (don't make the exception Permanent unless you find out how to undo it first - I don't know where to access the list of exceptions in Quantum [yet]).

If it indicates that DU has a valid certificate (as my instance of FireFox does), perhaps the connection is just timing out (although I'm not sure why that wouldn't be affecting Vivaldi too). You could try increasing the network.http.tls-handshake-timeout value in the about:config settings (just make a note of the current value so you can return to that value - mine is set at 30, which is the default). I'd probably just put TLS in the search field to see if any resulting preferences are not set to their default values.

Do you have another computer or mobile device to compare your ability/inability to connect to these problem sites?

BTW - the DU certificate says it uses TLS 1.2.

No love. Under Certificate Status it says : No Information Available. Unable to obtain identification status for this site. This is happening with Firefox, Edge and IE 11 but not Vivaldi. I do have TLS 1.2 checked in Internet Properties > Advanced > Security. The only other site affected (as far as my bookmarks are concerned anyway) is the one for my pre-paid debit card. Should I be concerned about that? No I don't have access to another machine, guess I could run up to the library later on but am puzzled by this. Oh, and the add exception box/tab is grayed out so can't do that either. Using same F.F. version you are.

Not a noob with computers but networking/internet is sorta my weak point. Any other advice? Thank you btw.

......was an overly zealous SSL scan from my A/V. Took forever to track it down but with some help from the Guru's at tenforums we got it.