Prevent Firebird's default instance from starting
There are two options


Manually change the startup type of the service
Remove (and optionally reinstall) the service 


Manually change the startup type of the service

You can disable it in the Services configuration. Use Windows-key+R or open run from the start menu, enter services.msc, find the Firebird instance (Firebird Server - DefaultInstance), right click, Properties and change the Startup type from Automatic to Manual:



Remove (and optionally reinstall) the service

You can also remove the service entry completely, to do this, you need to start the command prompt as Administrator. Under Windows 10 this would be Windows-key+X, A. I believe this doesn't work under Windows 8.1, there you may need to use Start, search for cmd or Command prompt, right click and select Run as Administrator.

In the command prompt go to the location of your Firebird install (or for Firebird 2.5 and lower, its bin-folder), and run

instsvc stop
instsvc remove


If you want to have the service entry, but just not automatically started, you can reinstall it with:

instsvc install -demand


This will install the service with startup type Manual (just using instsvc install or instsvc -auto will install it as Automatic); see instsvc for more information.