I personally use FB1.5.3 and want to eventually then add FK towards the database. It takes place when I attempt to get it done while other people is attached to the DB I recieve the best.

It is possible to method to add an FK as the product is running?

There is no way.

It is a known limitation that's been removed in Firebird 2.. You need to either upgrade, or accept it.

In my opinion this can be a limitation on firebird 1.5 and earlier.
Try google for (firebird foreign key "object is being used").