If they are connected to WiFi for pairing purposes in the Sonos App and you connect one via Bluetooth to the FireTV, then the audio will play to both in mono/stereo, but note there’s no guarantee that the Bluetooth connection will stay in lip-sync with the video on screen - you would be far better off with using a wired Sonos HT Product, such as the Ray, Beam, Arc etc. linked direct to the TV, as their wired connection is far more reliable.

Ken, thanks for the feedback.  One other question, is there a USB to audio input Y-Adapter that would allow the two speakers to connect to a TV audio output?  I really don’t need another “sound bar.” 

https://www.sonos.com/en-us/shop/sonos-line-in-adapter

There is also a combo adapter (ethernet+ line in) if you need that.  You would then use a headphone 3.5mm cable or adapter to RCA cables.  You only need to connect one of your speakers, not both.

However, like bluetooth, your likely going to run into lip sync issues as the Era 100 is built for multiroom wireless audio, where buffering the audio is needed.  They are not built to play in sync with TV audio.  The line in connection is primarily there for turntables, CD players, etc where there is no video content to sync to.

I’m using a pair of Era 300s connected to a TV via bluetooth.  However, the speakers are 99% of the time used for music and the content on that TV is always going to be background news, youtube videos, etc where I’m not too concerned about lip sync.  I would not plan on watching a show or movie that way.