My day-time job involves software development using Firebird database (which happens to be 20 years old this month). While it's a great little database that is easy to deploy and scales well, there still lacks documentation for developers (unless you go and buy the book). There might be some commercial desktop development/administration applications, but few quality free ones. Here are some that I am currently using.
IBOConsole has the look and feel of Borland's IBConsole, bundled with Interbase 6 open source edition. It is much enhanced, has almost all the features I need, and has good support for Firebird 1.5 (which Borland's IBConsole doesn't). You also need only one single .EXE to run it, which is very useful when you need to get it installed on a remote box to do some administration. Definitely my current favourite.
However, it is not free from bugs, but there is always a way to work around it. From the change log IBOConsole also has not been updated since late last year. The Delphi source code is there, and it is covered by MPL 1.1 - hopefully the development will continue again.
FlameRobin is a new cross-platform administration GUI for Firebird, and currently it is at 0.1.3. I saw it this morning when I wondered into Firebird's website, and immediately downloaded the Windows binary to give it a try. According to the developer guide page, it is implemented in C++ with wxWindows so that it can be ported to both Linux and Windows. The UI is relatively simple (but clean), showing its "alpha" status. HTML interface with customised protocol handler have been used to display "properties", but many functions have not yet been implemented. Hopefully it would be an active project.
One thing I really like about FlameRobin is its SQL auto-completion aka IDE style. Also I found its client library works quite well - IBOConsole barks at me when I try to get it to evaluate an UNION. Licensed as an open source project, FlameRobin is definitely something to watch out for.
IB_SQL is a free (as in beer) Interbase/Firebird utility, created by IB Object's Jason Wharton. It actually has a lot of useful utility functions, but I found it not intuitive to use. It might be someone's Swiss army knife, but just doesn't work for me.