> Thanks for looking into this, but, the thread that was stuck when the
> process was killed was executing a query that several other threads
> before it executed successfully and did not hang.

Can you be more specific about this query ? Does it update\delete
many records ? Does previous executions have the same set of input
parameters and database state ?

> Additionally, I have since reverted back to 1.5.3 again and have been
> running without any issues.

Did you tried FB 1.5.3 classic or super ?

> Is there possibly more information that I can send to dig in to this
> further?

Ideally if you can give me reproducible test case

Regards,
Vlad

PS I don't remember - did you tried to set GCPolicy=background
in firebird.conf ?