both drivers will be supported pdo_firebird is a lot easier to use in orm , but if you need the low level firebird api features then ibase is the way 

My vote is to go with pdo_firebird first (i saw that you have already started with it) and if is not enough then go deeper with ibase driver 

After you finish firebird driver and create a pull request you can ask for testers and we will announce on this list and firebirdnews / reddit ...

Also i'm interested in drupal8 driver for firebird