Currently:

• Sequelize officially supports PostgreSQL, MySQL, MariaDB, SQLite, Microsoft SQL Server, and DB2.
• Firebird (including Firebird 4.0) is not officially supported. There are some third-party dialects like [node-firebird](https://www.npmjs.com/package/node-firebird) or older forks for Sequelize, but they are often incomplete and may not support all modern features of Firebird 4.0.