display firestore vuejs

Create a posts array in your data:

...
data() {
    return {
        posts: []
    }
},
....
Then create a method that fetches the records and assigns the result to the posts array:

 methods:{
   getPosts: function() {

    fb.postsCollection.orderBy('createdOn', 'desc').onSnapshot(querySnapshot => {
        let postsArray = []

        querySnapshot.forEach(doc => {
            let post = doc.data()
            post.id = doc.id
            postsArray.push(post)
        })

        this.posts = postsArray;
    })

   }
 },
 .....
Call the method in the beforeMount() lifecycle hook:

 beforeMount(){
    this.getPosts()
 },
Then render the posts out to the DOM. For example:

<div v-if="posts.length">
    <div v-for="post in posts">
        <h4>{{ post.createdOn }}</h4>
        <p>{{ post.content}}</p>
        <ul>
            <li>comments {{ post.comments }}</li>
        </ul>
    </div>
</div>
<div v-else>
    <p>There are currently no posts</p>
</div>