According to NFC Record Type Definition (RTD): "The External Type Name is meant for organizations that wish to self-allocate a name space to be 
used for their own purposes.  [...]
The External Type Name MUST be formed by taking the domain name of the issuing 
organization, adding a colon, and then adding the type name as managed by the organization"

So 'mozilla.org:bug' is just an example of properly formatted type field for TNF External Type.