I found a few problems with this issue:

• I couldn't figure out how to label this issue, so I've labeled it for a human to triage. Hang tight.
• This issue does not seem to follow the issue template. Make sure you provide all the required information.

Thanks for providing the repro! What works for me is moving the initialization logic to beforeEach function. I don't exactly know how mocha works internally, but my guess is you need to provide the initialization code in the mocha context, and it does some cleanup for you.
Anyway I think the best practice for unit tests is using beforeEach for initialization. Hope it helps.

@Feiyang1 Thanks for looking into it and for providing a solid workaround - I really appreciate that.

A somewhat related issue, in case somebody stumbles upon this: If you use firebase.auth() you need to auth().signOut() so the node process can be terminated. So summarising @Feiyang1's solution - that's what worked for me

describe('', () => {
    let firebaseApp;

    before(async () => {
        firebaseApp = await firebase.initializeApp(config);
        return firebaseApp.auth().signInWithEmailAndPassword(mail, password);
    });

    after(() => {
        firebaseApp.auth().signOut();
        firebaseApp.delete();
    });

    it('....', async () => {
    });
});

@Feiyang1 would it make sense to perform things like signing out when you delete an app?

I'm glad it worked out for you. Can you please open a new issue regarding the signOut, so we can track it properly?