SSL-TLS Certificate Error when using Firefox v60 to v73

Beginning at the version 60 release of Firefox, TLS security settings have been incremented and will cause an issue with some websites that were not a problem for Firefox versions prior to this. The problem will be a certificate warning as shown in the first attachment below.

One alternative is to use a different browser.
Some options, in order of preference (dependent on O/S) would be...

Chrome (only versions less than v65.x, for v65+ see similar KB article)
Microsoft Edge
Microsoft Internet Explorer
Safari

But if you still wish to use Firefox then you can work around this issue as follows:

Use the configuration interface (*) of Firefox by typing in...
about:config

Then search for "TLS" and look for the parameter shown in the 2nd attachment which will be "4" before editing (as of this writing, for FF v60). Simply change this value to "3" and then close the tab. (do not include quotes in search or entries)

Try your site again and - assuming it was not due to a different cause - your site will be working again.

Summary of changes
security.tls.version.max 3

FOR FF74+
see This Article

(*) Do not make any other changes here unless you know what you are doing!