Note this answer is actually by @cincura.net on the Firebird .NET mailinglist. I am just posting it here for completeness:
The ADO.NET stack is not available in WinRT, hence you cannot use any database access. The CF version didn't helped either.
See How should a Windows 8 Metro Application connect to a central database? and How might a Windows 8 Metro app handle back end database access? for potential solutions.