There is an existing Bug after converting HSQLDB to Firebird. This deals with sub forms and the error you received. There is another error of this type dealing with a MySQL database. Both can be fixed by either running a macro or editing a file within the Base .odb.
I would recommend the macro and have created an extension to make it easier. See this post → How to unzip ODB file and edit the content.xml file to fix Firebird bug?. The related bug # is also there.
As with any fix, PLEASE make a backup copy first. Have run this myself numerous time without a problem but there is always a first time something can go wrong.
You can remove the extension when done if desired or leave it installed if you have other files to convert. It only need be run once to fix the problem.