Hi Woraphon -- Google's crawlers are now capable of indexing JavaScript sites, so for SEO purposes Firebase Hosting should generally be fine. Some other crawlers, such as Facebook's OpenGraph data and Twitter cards, however, do not crawl JS sites.
For these crawlers there isn't a great solution currently unless you can pre-compile your site into static files and upload those. We've heard this feature request a number of times and we are working to make Firebase Hosting more flexible for these use cases, though there's nothing specific to share at this time.
Cheers,
Michael