If I want to add security features to my application will it be possible to create the RBS tables in the same database (Firebird) or the RBS can only be deployed in SLQ Server / SQL Express?
Yep, you were clear. And no, it is not possible to use Firebird to house the RBS tables, as there are some features in it that will need SQL Server 2005.
If the latter is true, how can I manage this situation since the database server is a Linux box? (and I don't want to run another server with Windows just to house the RBS database).
Well, you do need the Windows somewhere. Usually, when using other databases, people just set up a computer for that matter. Is it a major concern in your environment to have a computer running windows? Maybe you could put a penguin on the top of it to make everybody happy... 
By the way, I haven't been to Argentina yet, would love to go and visit the country some day. Where are you over there?