Building Ruby extensions required using the same version of MSVC that had been used to build Ruby and another compiler could not be substituted, because numerous bug fixes and workarounds for Windows were specific to that one compiler. Building the Firebird extension required three different options to specify the location of various libraries, which in the tradition of Windows, had no standardized locations.
The workstation with the magical combination of build tools and libraries has long been retired, so while the Unix version of the extension has seen bug fixes and improvements, the Windows version has languished. It was generally good enough for development and small utilities, though and since my company deploys on Linux servers, there was no strong incentive to invest in a solution. But I always felt bad for the Windows users left out in the cold.
I'm happy to say that the RubyInstaller Team has done great work with MinGW32-based build of Ruby 1.8.7 and the associated DevKit. It only took a couple tweeks to extconf.rb to recognize the mingw32 platform along with mswin32 and then everything "just worked." Well, almost. You still had to build by hand, specifying --with-opt-dir=C:/Progra~1/Firebird/Firebird_2_1 (or wherever your Firebird installation was). To make gem installs work better, it now looks in C:\Program Files\Firebird for the latest version of Firebird and uses that one. Suggestions for improving this logic to work with foreign language versions of Windows are solicited.
In a more-perfect world, every Windows machine might come with a proper development toolchain, but for now you'll need to install DevKit in the proper location and have Ruby's bin directory in your PATH.
fb-0.6.6, with the latest tweaks, is on RubyGems.org and github.
UPDATE: fb-0.6.7 with improved installation for non-English Windows versions is out.