You have not chosen to trust "Entrust Certification Authority - L1C", the issuer of the server's security certificate.

Just randomly my Mac started giving me this error.  No matter how I connected to a Citrix Desktop, I got this weird error.  I looked at all the certificates in the browser and I looked at the certificates in the Keychain Access.  No where could I find "Entrust Certification Authority - L1C".

Well here is how I fixed it.

Here is how you would do it for a Windows Server (Link)

Here is how to fix it on OS X:

First download the cert here (Full Site)

Next open Keychain Access (Macintosh HD -> Applications -> Utilities - > Keychain Access)

Next click the lock up in the top left and enter your password

Next click file import, navigate to where you saved the certificate above, mine was called entrust_l1c.cer, and click the certificate.

Make sure at the bottom the Destination Keychain is selected to System, now click Open.

You will be prompted to type your password again one more time.

At this point, I tried it again and no more error.


Versions:
OS X 10.10.3
Firefox 37.0.2
Citrix Viewer 11.8.2