Fixed Database Rols are: db_ddladmin, db_owner, db_accessadmin, db_securityadmin, db_backupoperator, db_datawriter, db_datareader, db_denydatawriter, db_denydatareader + one more that is dbm_monitor.
This is not a new Fixed Database Role, but you can use it when you establish the database mirroring on your SQL Server. It will be available in msdb sys database, at the moment when the first database is registered in Database Mirroring Monitor. This database role has no users asiggned, so the sys admin must assign the user to this role.
When you are member of the dbm_monitor database role, you are granted to view any database (mirroring), also when you connected to the server you will have limited permissions on the instance where you would be able to view the most recent status in Database Mirroring Monitor.