Here are a few resources for you to consider:
- Sphinx very powerful and popular free open source full-text search engine.
- Textolution Fulltext search for Interbase and Firebird.
- IBObjects Full Text Search ("Fuzzy Search") module, a fully working module that can be used to set up your search indexes or as a model for your own custom implementation.
- Rubicon is a Delphi add-on that lets you put full text search capabilities into your applications.
- Fulltext Search for Firebird SQL By Dan Letecky on CodeProject using DotLucene full-text search engine.
- Mutis is a Delphi port of the Lucene Search Engine. Provide a flexible API for index, catalog and search text-based information with great performance. Excelent for implement custom search engines, researching, text retrieval, data mining and more.
There is a fork of Firebird code made by a company called Red Soft. It's licensed under the same license as Firebird, so you can take a look at their version which can support full-text searches using Lucene engine via JavaVM interfaces.
You can also read a paper titled "Full text search in Firebird without a full text search engine" by Bjoern Reimer and Dirk Baumeister, presented at 4th Firebird Conference.