Firebird is an open source database which is pretty popular. Own of the downsides of open source stuff is imho the fragmented support. Firebird is a good example of this. You can use it in Visual Studio 2005, but all documentation is mainly directed inward and deals with working on the Firebird for .net project and not on using a Firebird database in your app. Googling around mainly leads to this blogpost or (partial) copies of it's content. These contents might be OK for developing the FireBird support but are both in- and over-complete. As a first step it tells you to install the Visual Studio SDK. Don't do this!  It has several downsides. You can only find it under MSDN subscriber downloads, there is a risk you'll download the wrong version (there's still a Whidbey beta floating around there) and you'll arrive in a licensing fog. Not quite open source :)

This is what does work:

• Download the ADO.NET 2.0 Firebirdclient Make sure you have a very recent copy, an essential update was published quite recently.
• Install it
• Register the FireBird provider in machine.config, the instructions are in the ReadmeDDEX file, found in the Firebirdclient directory.
• Edit the FirebirdDDEXProviderPackageLess registry file, found in the Firebirdclient directory.
• Register the FirebirdDDEXProviderPackageLess registry file.
• Check that all setting in the FirebirdDDEXProvider registry file are deleted from the registry. Else VS will slap you with error messages.

Now you will see Firebird in the list of dataproviders of Visual Studio.

Many thanks to Firebird afficionado Leon, who was stubborn enough to try every variation and got things working.