Merge Modules for Firebird 2.1.0 have now been released and are available from here. Firebird 2.1.0 is a major new release. Please note that these merge modules were updated at 16:10 GMT 2008/5/15 to correct a dependency error in the fb tools merge module.
Current releases of Firebird 2.0.4 and 1.5.5 are still available.