You will need to setup a reverse proxy that handles ssl and letsencrypt in front of the syncstorage-rs docker and make sure you don't bind to http://localhost/ but https://example.com/.
Firefox clients refuse to connect to a sync storage that doesn't use a signed certificate unless it is on localhost.

To use it on firefox mobile you will need to activate the developer mode by going to Settings -> About Firefox and press the logo multiple times (I think five?). Exit the settings menu and enter it again. Now you will see Sync Debug at the top of the settings menu and in that menu you can setup custom servers.