When you have a lot of users on your server performance may be an issue. Performance Analyzer can be used to monitor database performance. While it can be used with any version of Interbase and Firebird the detailed and most accurate results can be obtained with Interbase 7.x only. Interbase 7.x has system temporarily tables with a lot of internal data about server state, running queries, connected users and so on. So the monitoring ability is much better with Interbase 7.x and higher versions.
Here is a screenshot of standard monitoring used for Interbase 6.x and Firebird: