What ADO drivers (not .Net) exist for Firebird?
Take a look at one of these websites:
http://www.ibprovider.com/eng/
http://praktik.km.ua/
there is an example how to use it with Visual Basic:
http://praktik.km.ua/eng/exam.html
Update: website praktik.km.ua is no longer available, but you might have luck looking at the snapshots in Internet Archive. For example, this one from year 2008:
http://web.archive.org/web/20080530202753/http://www.praktik.km.ua/
If you wish to use Firebird with Visual Basic, you might also want to take a look at this:
http://fbdll4vb.sourceforge.net/
It's a stand-alone DLL library without any dependencies.