PositiveSSL not valid on firefox4

Hi Guys,

Ive got a positiveSSL cert installed on my site.
It works fine for chrome, Ie, and firefox ( below v4.0 )

However on firefox 4.0 it says its an untrusted cert.
(Error code: sec_error_unknown_issuer)

The cert is installed on:

Its running on apache linux.

Any ideas whats wrong here? or is comodo no longer a trusted supplier?

Have you installed ALL the chained root certificates from Comodo?