It would be completely unsupported but the jdbc_response probe, while it is intended to only work with the 4 or 5 preconfigured drivers, can be coerced to work with another open jdbc driver assuming firebird has one. The trick is to use a connection string that would be valid with one of the supported databases but targets the firebird db connection too. That leaves the probe thinking it is talking to Oracle (for instance) but passing commands to the db you have configured.
One of the many other alternatives is to write your queries in a script and use logmon to call them. And parse the data from there. Obviously, you are left needing to implement all the error handling yourself but that isn't that onerous.