SvelteKit + firebase example in $layout.svelte

$layout.svelte

<script>
  import "./styles.css";
  import { session } from "$app/stores";
  import { onMount, setContext } from "svelte";
  import { writable } from "svelte/store";

  let store = writable({});
  setContext("store", store);
  $: user = $session.user || null;
  onMount(async () => {
    window.ssrUser = user;
    const storeModule = await import("$services/firebaseClient");
    store.set(storeModule);
  });
</script>

<slot />