What's the effort emigrate from Blackfish to Firebird ?
I personally use DBX4
No storeProcedure, or function or sights
I access my data trough SQL queries
If you do not depend on specific Blackfish features (SP, trigger, admin packages calls etc), the modification should not be any discomfort.
I have did this type of migration inside a DEMO for any customer.
For those who have used Blackfish like a "assortment of tables accessible via SQL" then your change won't be difficult whatsoever.
There's a person for delphi and dbx4 firebird http://sites.google.com/site/dbxfirebird/
my prediction is that's simple for you to dump the blackfish tables into csv after which to import them in firebird and then modify just the application to gain access to the firebird db while using dbx4 firebird configurations