How to protect data in Firebird database?
Short answer: use encryption
Firebird does not have any encryption integrated, but there are various solutions you can add.
One is to encrypt all the data on the client before saving to database columns, and decrypt while reading. The only problem with this is stuff like indexing and searching. While indexing on foreign and primary keys will still be effective, searching the data with STARTING WITH or numeric operators (less then, more than, etc.) won't be able to use index as you would have to decrypt the data before you can compare it.
Second solution is to encrypt the filesystem where Firebird database is stored. There are various solutions like VeraCrypt, EncFS or BestCrypt that do this. Since the product pages contain only the advantages, we are listing their Wikipedia pages where you can find more reliable information about disadvantages and possible problems of each system:
If you only use Linux, there are various tools and libraries for this. Take a look at these articles for some examples:
A popular encryption system that comes pre-installed on Linux is DM Crypt, which is part of the kernel:
Possible problems with this approach are slower access to database and the fact that the unencrypted database is readable while your application is running.
Third, since Firebird is open source, you can alter the code that reads and writes pages to the disk to encrypt and decrypt them. Of course, you would have to find a suitable way for client to send the decryption key to the engine. Please note that this means that all users use the same key.