Is there some Data Dictionary tool or feature in Firebird?
Yes, there is. All database objects are stored in system tables (names starting with RDB$) and almost all of those tables have RDB$DESCRIPTION field, which is a textual BLOB field, so you can put unlimited amount of text there.
Most GUI administration tools give you a way to easily edit those description fields (for example, each properties page in FlameRobin shows those descriptions by default). There is also a specialized freeware tool called IBDesc that is also able to print nice reports about entire database: tables with fields and descriptions.