Here's list of tools for Firebird on their website
http://www.ibphoenix.com/resources/projects/
There are plenty of good ones, both open source and commercial. Some are limited solely to Firebird, some can work with many different databases (i.e, MySQL, MSSQL, and Firebird all with same tool), and some are cross platform and/or Java- and/or web-based.
DBWorkbench is a first class commercial product that could work with MySQL, MSSQL, and Firebird. They have a free "lite" version that's restricted to one db, you might check out that one for Firebird: