Firebird 3 is focused on performance and security. Many changes were necessary to accomplish this, and some of those changes may confuse some users coming from older versions. The Migration Guide to Firebird 3 eBook came to help existing users to understand those changes, and migrate their servers/databases to Firebird 3, avoiding headaches and in the shortest time possible. More information about the English version of the eBook can be seen here.
For Brazilians, there is a version in Portuguese here.