I found some "solutions":

http://bdn.borland.com/article/0,1410,25171,00.html

Unfortunately, most of them have a problem with either table scanning
or duplication of indexed data stored in upper case. Of course, having
to table scan for items your searching for is an unacceptable
solution. Duplicating data also stinks because if you ever decide to
add a string field to an index, you have to change your data model by
adding a new field to the table. Not good.

The link to brookstonesystems showed that they currently offer
case-insensitive collation for Windows only. I need it for a Linux box.

Thanks for the help, but it looks like Firebird isn't the solution I
was looking for...