I have JForum working on Firebird 2 database. And all looks well.
If any one is interested, I'll hand over the code but if not, then I'll continue testing etc.
For those of you that do not know Firebird, it is:
Holds UTF-8 encoded data.
Fully SQL compliant with db procs, triggers etc.
Very easy to set up and maintain.
I have used it for over 7 years and it's never crashed on me.
My opinion of JForum is that it's a great forum, I've really enjoyed using it.