Commented by: Sean Leyne (seanleyne)

The current functionality is "as designed" within the current engine design/limitation.

Previously, you couldn't perform FK operations if any users was connected. Now, you can perform the operations with connected users.

What is, perhaps, not explicitly stated is that the object (any object) can't be referenced by a current connection/prepared statement.

Commented by: @hvlad

This is support question and should be asked in support list

Creation of FK requires PR (protected read) locks on both relation's metadata.
I.e. if any if two relation is used by active concurrent transaction and this transaction hold PR or EX lock on relation's metadata, FK creation will fail.
Ask for more details in support list, if you really need it ;)

Commented by: Felipe Jaekel (fkj)

This link could be updated too http://www.firebirdfaq.org/faq22/. It also gives the impression that only earlier versions of Firebird had this limitation.

Can it be considered as a "Improvement" or "New Feature" then?

I face this problem since version 1.5, and disconnecting all users from database to maintain foreign keys is too stressful.

Thanks