For example, Firefox users have problem to visit my HTTPS-sites.
The “SSL-Checker” (https://www.sslshopper.com/ssl-checker.html) reports:
The certificate is not trusted in all web browsers. You may need to install an Intermediate/chain certificate to link it to a trusted root certificate. Learn more about this error. You can fix this by following GeoTrust’s Certificate Installation Instructions for your server platform (use these instructions for RapidSSL). Pay attention to the parts about Intermediate certificates.
One of the certificates is signed with a SHA1 signature. We recommend that you reissue or replace this certificate with one that uses a SHA-2 signature. Contact your SSL provider about how to do this. Read more about the SHA-1 deprecation here.
So how can I or Dreamhost fix it?