Acerca.js:

import React, { useEffect, useState } from 'react';
import { db, onValue, ref } from './Firebase';

const Acerca = () => {
  const [feature, setFeature] = useState();

  useEffect(() => {
    let featureRef = ref(db, 'feature');

    let unsubscribe = onValue(featureRef, (snapshot) => {
      setFeature(snapshot.val().feature);
    });

    return () => unsubscribe();
  }, []);

  return <>{feature ? <h3>{feature.title}</h3> : null}</>;
};

export default Acerca;

Firebase.js:

// simulate real firebase module
export function onValue(ref, callback) {
  const snapshot = {
    val() {
      return 'real implementation';
    },
  };
  callback(snapshot);
  return function unsubscribe() {};
}

export function ref(db, name) {
  return {};
}

export const db = {};

Acerca.test.js:

import React from 'react';
import { render } from 'react-dom';
import { act } from 'react-dom/test-utils';
import { onValue } from './Firebase';
import Acerca from './Acerca';

jest.mock('./Firebase');

describe('Acerca', () => {
  test('should pass', async () => {
    const data = { feature: { title: 'New feature' } };
    const snapshot = { val: () => data };
    onValue.mockImplementation((ref, callback) => {
      callback(snapshot);
      return jest.fn();
    });
    const container = document.createElement('div');
    await act(async () => {
      render(<Acerca />, container);
    });
    expect(container.querySelector('h3').textContent).toBe(data.feature.title);
  });
});

test result:

 PASS  examples/69324329/Acerca.test.js (8.029 s)
  Acerca
    ✓ should pass (20 ms)

-------------|---------|----------|---------|---------|-------------------
File         | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
-------------|---------|----------|---------|---------|-------------------
All files    |      70 |      100 |    37.5 |   73.68 |                   
 Acerca.js   |   91.67 |      100 |      75 |     100 |                   
 Firebase.js |    37.5 |      100 |       0 |    37.5 | 2-8,12            
-------------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        8.556 s