Openfire currently uses Proxool - https://github.com/proxool/proxool - to pool database connections. This library is no longer actively maintained and it's use is preventing the use of the Microsoft SQL Server JDBC drivers - see OF-1404.
Apache Commons DBCP - http://commons.apache.org/proper/commons-dbcp/ - is a widely used alternative, and Openfire should switch to that instead.
NB. Targeting for a post 4.2 release, following discussions on open chat.