My hosting provider has recently [re-issued and re-installed an SSL certificate](https://serverfault.com/questions/769176/how-long-does-it-take-to-renew-an-ssl-certificate/769422#769422) for my domain, after they let the old one expire by mistake. 

I am now able to browse [the website](https://bilingueanglais.com) over HTTPS again, and so is my host, and so are a number of other users.

However, *some* users (at least a dozen out of hundreds) are still getting `Your connection is not secure` error messages on different browsers and platforms. (It is proving difficult to diagnose an issue I cannot reproduce.)

I understand different browsers use different lists of Certification Authorities (CA.)

1. How come a user running the same version of Firefox as I am (45.0.1 on OS X) is getting a `SEC_ERROR_UNKNOWN_ISSUER` error (for my site only) while I'm not? What makes it possible? Said user cleared his cache and rebooted his laptop.

---

I ran an [SSL check on digicert.com](https://www.digicert.com/help/). The result is this:

> 
> SSL Certificate is not trusted
> 
> 
> The certificate is not signed by a trusted authority (checking against
>  Mozilla's root store). If you bought the certificate from a trusted
>  authority, you probably just need to install one or more Intermediate
>  certificates. Contact your certificate provider for assistance doing
>  this for your server platform.
> 
> 
> 

2. How come I am able to connect to the site without SSL error if this is the case?