One branch of Firebird is an Oracle clone called Fyracle which is even able to run Compiere. On this page you can find reasons why authors chose Firebird over PostgreSQL, which might also help you with your decision. How successful this Oracle emulation is can be seen from this quote:
Firebird-Fyracle supports the major
Oracle-based ERP/CRM application
"Compiere" -- with virtually no change
to the Compiere code base. Fyracle has
supported Compiere since version
2.4.2b, which contained over 20,000 lines of PL/SQL. The current release,
2.5.3c, has moved most PL/SQL into stored procedures written in Java,
which is also supported by
Firebird-Fyracle.
