In order to keep the server low reasonable, you might want to limit the time a single query can consume. Firebird does not support this directly yet (there are plans for Firebird 3.0).

However, you could periodically query the monitoring tables (Firebird 2.1 and above) to detect and cancel long running queries. You can do:

SELECT * FROM MON$STATEMENTS;

Look for those having MON$STATE set to 1.

Please note that your database needs to be at least ODS 11.1, i.e. created with Firebird 2.1 or above. Older databases won't show you these tables even if you use Firebird 2.1 to access them. To learn more about ODS and how to retrieve it.