Firebase has recently announced support for the i18n rewrites feature. It allows you to serve different content depending on a user's country or preferred language.
But since the different versions of the content are served under the same URL, a search engine crawler will only be able to find one version of localized content.
Without i18n rewrites, I would need to create a root page that redirects to localized ones depending on the user's language preference or location (myapp.com redirects either to myapp.com/en/ or myapp.com/fr/). With that approach, crawlers could easily find and index all localized versions of my website.
But how can I achieve that discoverability with i18n rewrites?
You should be able to use a combination of <link rel="alternate"> and <link rel="canonical"> for this. Firebase Hosting will still serve your localized content from the full path (e.g. if your root is /intl and someone visits /intl/fr they will see the French content regardless of headers and cookies).
The Google crawler will use local IPs from a variety of countries but no Accept-Language header when crawling (source). So my recommendation would be:
<link rel="alternate" langhref="{lang}" href="{url}"> tags for each localized version, where {url} is the absolute URL to the localized version (e.g. /intl/fr/about.html).<link rel="canonical" href="{url}"> that points to the full localized version. (see Google guidance)With those tags in place, Google should be able to discover the multi-language and multi-locale content in your site.