Does Firebird support full-text search?
Not directly in the engine, but there are 3rd party solutions:
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: