Well my experience with Heroku has always been that the code barely cares that it's being hosted there: a few config/environment variables here and there and that's it. Nothing to do with Firebase, where your project structure, front-end code, scripting utilities, config files, etc. become coupled with Firebase even if you try your best to contain the coupling.

I'm interested, what does Heroku offer in the way of "local testing" anyway? I just use a web framework's normal test runners in my local shell.