Looks like you are not new to certificate drama. https://support.mozilla.org/en-US/questions/1270495

Just a guess, but I would say try going to the certificate manager and import them. (☰>settings and search for cert)

Thank you Matt ! I had completely forgotten that previos question in Nov, Dec 2019. Obviously, I have had no problems since then, though I have had several such certificates. I think you are suggesting that I try to download it from the "thesslstore" site. I shall try that and let you know how it goes.

I have asked the ssl people. This is what I said to them. " The download talks about downloading to a server. I am an individual with a Windows 11 Pro laptop using Firefox and Thunderbird. No server. I think I would like to be told haw to convert the 3 .crt files you sent me to a .pfx file Please. "

They want to know whetther I have the privaye key. I think I thought that Thunderbird did produce one and incorporated the public key in the CSR file. What I do not know is how Thunder bird was expecting the certificate provider to return the certificate. Any ideas please ?