Why doesn't my query plan use index?

There could be various reasons:

Make sure that your query has WHERE, JOIN or ORDER BY clause involving the indexed table columns, otherwise the index is useless and won't be much benefit using it.

If you do have index, but it isn't used, perhaps you have ascending index (default) and you need descending (or vice versa).

Also, your index statistics can be out of data, You need to rebuild the index statistics, so that Firebird finds it usable. That can be done with SET STATISTICS sql command.

If you use views with unions (you cannot index a view), I highly recommend you use at least Firebird 2.0 as earlier versions don't use any indexes of underlying tables when you use WHERE or JOIN with a view. If you can't use Firebird 2.0, the only way to speed it up is to write a stored procedure that takes value in WHERE clause as agrument.