That's interesting indeed !
Unfortunately, I do not use the Adobe application you name and, therefore, cannot give you detailed information about the interaction that may exist between it and the Keychain...
The first thing to do would be to create a new backup file from Mozilla Firebird in order to make sure that the one you are using is not corrupted in any way. Then, try to open it in the keychain by using different methods : double-click, drag-and-drop, changing the default application in the "Get Info" window"... Indeed, these methods usually lead to slightly different results.
Also, S/MIME functionality has been added to Mail in Mac OS X v. 10.3 Panther. It is possible that Mac OS X v. 10.2 Jaguar handles such files differently.
Let me know if this helps,