Hi @Srvmj
Welcome to the Sonos Community!
Good question!
The only protocols that work remotely in that fashion are AirPlay and Chromecast, and, as Firesticks are made by Amazon and not Google or Apple, neither Chromecast (not supported by Sonos anyway, and a Firestick would be more likely to be set up to receive Chromecast rather than to send it) nor AirPlay are supported.
As for lossless audio, there would not be any point - whatever you stream from an online source on your Firestick is not going to be using lossless audio in the first place, so there would be no point in using lossless locally.
I am not overly familiar with Firesticks - does yours support Bluetooth? If so, then you can already connect it wirelessly to your Era 100 speakers, but please be aware that it may not sync properly with the picture as Era 100s are not designed to be Home Theatre primary devices (as our sound-bars are).
I hope this helps.
Using my FireStick (4k and 4K Max) I am able to stream from it to my TV in both Atmos and UHD using the Amazon Music app and have no issues streaming Atmos or Dolby to the TV.
As far as I can tell Bluetooth 5.2 + BLE is supported for audio output, Headphone and speaker pairing is offered on the Bluetooth menu. No details on formats supported.
https://developer.amazon.com/docs/fire-tv/device-specifications-fire-tv-streaming-media-player.html?v=ftvstick4kmax_gen2_16
Be fun to try and pair a Sonos device with Bluetooth input to one just to see what happens. Almost wish I had something that would do that.