Remove Firebird seperately via the built-in uninstaller / from the windows control panel.
If that doesn't work or if it's not listed there, but still has residue left on your computer, you can either get in touch with the Spacial helpdesk or the Firebird community.
Honestly, nobody in this forum knows anything but the very basics about Firebird.
Firebird is nice for small applications that need structured data, but once Firebird crashes you're out there on your own. That's why advanced users will always recommend MySQL over Firebird.
Never seen that error message about MySQL. I don't even know why it would check if MySQL was installed. There's no need for that since MySQL could very well run on a completely different machine.
-nt.exe is the MySQL service file, but any complaints about that should only ever come from MySQL itself and not from SAM.
Which version of MySQL did you install? Did you enter the proper credentials in the SAM installer config dialog?
I once did a tutorial video on how to switch SAM to MySQL: https://www.youtube.com/watch?v=kJU5f3nZYtI
This is a bit outdated, mostly because the way to find the correct MySQL open source installer has changed, but the SAM configuration stuff should still look pretty much the same. Maybe that can help you find where you took the wrong turn...