Starting with Firebird 2.0, the fbclient.dll must also be distributed with Visual C++ library dlls. These should be placed in the same folder as fbclient.dll (or whatever it is renamed to).

For Firebird 2.0.x, the Microsoft redistributable "msvcr71.dll" is required and must be placed in the same folder as fbclient.dll.

For Firebird 2.1.x,  the Microsoft redistributable "msvcr80.dll" is required instead and must be placed in the same folder as fbclient.dll. Additionally, the Microsoft redistributable "Microsoft.VC80.CRT.manifest" must also be installed alongside msvcr80.dll if the installation is to work on later versions of Windows.

Note that fbembed.dll also requires support dlls. These are all included in the zip file distributed by the Firebird SQL website.