...

  fetchUsers({ commit }) {
    commit('initUsers')

    return new Promise((resolve, reject) => {
      // 以下の行を変更
      usersRef.where('age', '>=', '25').get()
      .then(res => {
        res.forEach((doc) => {
          commit('addUsers', doc.data())
          resolve(true)
        })
      })
      .catch(error => {
        console.error('An error occurred in fetchUsers(): ', error)
        reject(error)
      })
    })
  }

...

usersRef.where('id', '==', payload.id).get()
.then(snapshot => {
  snapshot.forEach(doc => {
    // 処理
  })
})

usersRef.where('age', '>=', '25').order('created_at', 'desc').get()

Uncaught (in promise) FirebaseError: Invalid query.You have a where filter with an inequality (<, <=, >, or >=) on field 'age' and so you must also use 'age' as your first Query.orderBy(), but your first Query.orderBy() is on field 'created_at' instead.

usersRef.where('name.first', '!=', 'Jane').get()

Uncaught (in promise) FirebaseError: Invalid value "!=" provided to function Query.where() for its second argument. Acceptable values: <, <=, ==, >=, >, array-contains, in, array-contains-any

docs.some((doc, index) => {
  if (doc.name.first === 'Jane') {
    docs.splice(index, 1)
    return true
  }
})