T530 Thinpro thin client will not store security exeptions

I am trying to set up a t530 Thin client running Thinpro, as a Citrix client using the web browser (Firefox) as connection. Since the site is set up using SSL and (for  reasons not to be discussed here) I am not able to install certificates in the t530, I have to add a security exeption for this specific site. But on the t530 (in opposite to its predecessor t510), the security exeption is not stored. When I close Firefox and try again, I get the security message and have to add the exeption again... Is this done on purpose? Or is it a flaw? Running Thinpro 6.2.0 build 22