Check the reason it is untrusted. Modern browsers encourage use of Subject Alternate Name entries to identify the identities (DNS domains, IP addresses, etc.) that the certificate should be used for. If you aren't referencing the site with one of these, you may have trust issues.
Some browsers require you to specify what you trust the certificate for. If you import the certificate and don't flag it as trusted for web sites, it will not be trusted for web sites.
The simplest approach for a single site is to add an exception for that site. The dialog to do this should be on the pop-up that says it is untrusted.
There are modern mechanism designed to prevent various security issues such as man-in-the-middle and certificates from an unauthorized authority. If they have had them enabled on the server, the current certificate may ve temporarily (up to a year) untrusted.