Firebase just had a recent update that included this functionality. You can use it like so in your firebase.json:
"rewrites": [ {
"source": "**",
"destination": "/index.html"
} ]
This is the code sample used in their docs and will send any directory or file that is not found back to index.html.
It's important to note that you will need to update your firebase deployment tools to version 1.1.0 or higher for this to work:
$ npm update -g firebase-tools
You may need to use 'sudo' with this depending on your permissions.
You can read the documentation here: https://www.firebase.com/docs/hosting/guide/url-redirects-rewrites.html
You can read about updating firebase tools here: https://www.firebase.com/docs/hosting/guide/command-line-tool.html