Oh, it was just right. I was investigating it. 
After updating ZeroSSL with Caddy, your Firefox may still be returning an error.
The old certificate is maintained somewhere. You can work around this with Firefox settings:
≡ - Settings Left sidebar 
Privacy & Security
Scroll down “Certificates”
Query OCSP responder servers to confirm the current validity of certificates 
to Uncheck
Of course, you can also use Chrome or Edge as a workaround.