https://webcompat.com/ to report. (Also in the menu: "Report broken site".)
Firefox
/c/firefox
A place to discuss the news and latest developments on the open-source browser Firefox.
Rules
1. Adhere to the instance rules
2. Be kind to one another
3. Communicate in a civil manner
Reporting
If you would like to bring an issue to the moderators attention, please use the "Create Report" feature on the offending comment or post and it will be reviewed as time allows.
Who are these people? What can they do?
They are the collective main browser makers, including Mozilla. One thing Mozilla can (and does) do is insert site-specific workarounds (e.g. changing the User Agent), and/or work with the website to ensure compatibility.
Yeah, it's right next to the wall of e-commerce sites that give you a single checkbox for "send me order information and tracking updates along with occasional promos"
I think the streaming with Xbox Cloud Gaming does not work on Firefox and requires a Chromium based or Safari browser. I believe the reason is because Firefox does not have an API for streaming games? Or the current implementation is not compatible with it. I read stuff like that in the past couple years and don't actually know why Microsoft does not support Firefox. So that would be a thing to put on a WOS: Wall Of Shame.
I rarely run into such sites, when I do I leave.
I've never run into one that I must use.