Two obvious (to me, anyway) questions: Have you verified that the same imapd binary is running on both the non-SSL port (port 143) and the SSL port (port 993)? If not, perhaps you modified one imapd binary to return a different view of the namespace, but not the other one.
Have you asked the Mozilla people? This seems to be more of a Mozilla issue than a c-client issue. imapd does not unilaterally send data; it has to be asked by the client (Mozilla in this case) to do so.
