Firefox has the StartCom root certificate.

• Tools > Options > Advanced : Encryption: Certificates - View Certificates

Are you sending all required intermediate certificates?

• http://www.networking4all.com/en/support/tools/site+check/

Many thanks cor-el for your reply, I have checked the site URL using the link you posted above. All seems fine but firefox still displays the same "untrusted" message.

The answer to your question is yes. I have downloaded all relevant intermediate certificates and uploaded them in one single bundle. Firefox still doesn't like it.

The site's URL is: www.saversquare.com

No problems here with that website.
I've seen such issues before and it helps in such cases if you remove the stored intermediate certificates that are used by that connection, in this case the "GeoTrust DV SSL CA".

Stored intermediate certificates show as "Software Security Device" in the Security Device column in the Certificate Manager.

• Tools > Options > Advanced : Encryption: Certificates - View Certificates

You see no problem now because after trying everything I possibly could with the StartSSL certificates, I ditched the idea of a free cert and opted for a new GeoTrust certificate, got it in a couple of hours and I had the site up and running in all browsers in no time.

There are other sites I am managing that use startssl where the problem still persists.

Here's a link to another example: [CAUTION: CONTENT WARNING]

https://www.itickle.co.uk/

You have a problem with implementing the StartSSL keys on your server. To test use this command: echo GET | openssl s_client -connect www.itickle.co.uk:443 -state -showcerts

Basically it says you have imported the wrong StartSSL root public key on your server.