This article from The Verge[0] cites some support Tweets:

- AirBnb[1]: "We'd always recommend that you use Google Chrome to browse the site: we've optimised things for this browser. Thanks."

- Seamless[2]: "Safari is not a good browser to use with our website. Google Chrome work better with our website or you can download the app."

- DirecTV Now[3]

- IDAT solutions[4] (some random thing I found while searching)

- Countfire[5] (ditto)

Groupon said something similar as well but walked it back. Whatsapp's web app didn't support Firefox initially but does now.

Could browse through Web Compat[6] as well, though I'm not sure if this site is more about sites that should work in FF but don't, or Firefox bugs.

[0]: https://www.theverge.com/2018/1/4/16805216/google-chrome-onl...

[1]: https://twitter.com/AirbnbHelp/status/752829250198245376

[2]: https://twitter.com/Seamless_Care/status/942798540396474368

[3]: https://www.theverge.com/2017/6/7/15758274/directv-now-googl...

[4]: https://www.idatsolutions.com/faq/

[5]: https://www.countfire.com/

[6]: https://webcompat.com/issues