The DAO mechanism should make this fairly easy. To do this correctly, you will need to create your own "FireBird" DAO package. Start off with all the classes being just simple subclasses of the GenericDAO implimentations.
You can then also start creating your own firebird.sql queries in association with this new DAO. If you can "tweak" the SQL to work, then the code can stay the same. Otherwise, you may have to modify the specific DAO methods as needed.
Probably the biggest "gotcha" in getting jForum to work with a DB is the use of Limits and Offsets in some of the
queries. Not all DB's support them. This is the main reason that it's hard to get jForum to support older MS SQL versions.
Of course, the other issue is how to test your new DAO... unfortunately, this is a pretty manual process. I'd suggest having access to a supported DB (e.g MySQL) so you can easily compare query results for differences.
Finally, all this is about 2.1.x.. jForum 3.0 uses hybernate so supporting Firebird should be as simple as changing the SQL dialog (or what ever hybernate calls it now). [originally posted on jforum.net by monroe]