This thread looks to be a little on the old side and therefore may no longer be relevant. Please see if there is a newer thread on the subject and ensure you're using the most recent build of any software if your question regards a particular product. Otherwise - post away!
The code generated by the NHibernate Designer is database agnostic, so you can absolutely use the NHibernate Designer to create classes and mappings, and you can then use those against a Firebird back-end in the usual way.
However, the NHibernate Designer does not currently support dragging tables onto the design surface from Firebird, or updating a Firebird database schema from the model. This is because, as far as we can tell, Firebird integration into Visual Studio is extremely sketchy at the moment -- there is a DDEX provider but it hasn't been updated for two years and installing it requires you to manually merge the machine.config and registry entries. You would instead need to set up the entities using the Toolbox as if you were building the model from scratch.