In firebird 2.5 backup security database:
gbak -user sysdba -pas masterkey -b {host/path}security2.fdb security.fbk

In firebird 3 restore copy of 2.5 security database:
gbak -user sysdba -pas masterkey -c security.fbk {host/path}security2.5
And run users upgrade sql script:
isql -user sysdba -pas masterkey -i upgrade25security30.sql {host/path}security2.5

Script will create users with new random passwords and type them to you.
It's your responsibility to notify users about new passwords.
