I assume you have installed FireDaemon v0.07b by Formida which can be downloaded here

Well I suggest you to read the entire Formida's doc for installing G6 FTP Server as System Service.
Personaly i have succeeded to install it as system typing

c:\>%MXBIN%\firedaemon.exe -p
FireDaemon v0.07b (c) 1999 Formida Software Corporation

This process will install a new FireDaemon service for you.
Please answer the following questions:

Service Name (no spaces): G6ftpserver
Application Working Directory (eg. d:\path): c:\program files\g6ftpserver
Application Executable (eg. d:\path\filename.exe): c:\program files\g6ftpserver\g6ftpsrv.exe
Application Options (eg. /opt1 /opt2): -h -s
Service successfully installed!
Press Enter to continue.

Then i have typed :

c:\net start g6ftpserver
Le service FireDaemon Service: g6ftpserver dΘmarre
Le service FireDaemon Service: g6ftpserver a dΘmarrΘ

That's all.

Now i wanted to have it starting at boottime, so i clicked on service in configuration panel :


(Well here it is in French but i think you can understand)

Extracts from Formida's FAQ:

"How can I make my service visible (for debugging)?"

Go to the Services applet in the Control Panel. Find the appropriate Fire Daemon service. Click on the Startup button. This dialog box has two sections: "Startup Type" and "Log On As". In the Log On As section choose the "System Account" option and check "Allow Service to Interact with Desktop". You will then need to stop and restart the service.

"I want my service to start as a domain user. How do I do this?"

Go to the Services applet in the Control Panel. Find the appropriate Fire Daemon service. Click on the Startup button. This dialog box has two sections: "Startup Type" and "Log On As". In the "Log On As" section choose "This Account" and type in the domain account you wish to use (eg. FORMIDA_A-P\JamesB) plus the domain password. If they are correct the user will be given the Log On As A Service privilege. You will then need to stop and restart the service.