I expanded my apache config:
Before (not working in Firefox):
SSLCertificateFile /etc/letsencrypt/live/exampl.es/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/exampl.es/privkey.pem
Now (working with all browsers):
SSLCertificateFile /etc/letsencrypt/live/exampl.es/cert.pem
SSLCertificateKeyFile /etc/letsencrypt/live/exampl.es/privkey.pem
SSLCertificateChainFile /etc/letsencrypt/live/exampl.es/chain.pem
Thanks