The server is started and stopped by 'fbmgr' executable from 'bin' directory of your Firebird installation. It is called 'ibmgr' in Firebird 1.0. To start the server type:

/opt/firebird/bin/fbmgr -start

To start the server with Guardian (Guardian watches the server and restarts it if it crashes) type:

/opt/firebird/bin/fbmgr -start -forever

To stop a running server, type:

/opt/firebird/bin/fbmgr -shut -user SYSDBA -pass *****

To force a shutdown, type:

/opt/firebird/bin/fbmgr -shut -force -user SYSDBA -pass *****

If you use Firebird 2 or higher, you can also use the regular 'kill' command to shutdown the server, as it handles the signals properly. Make sure you first kill the guardian and then the server (otherwise guardian would restart the server).