Check the browser engine Media Center is using. If you're using the Internet Explorer option, there's a good chance the website has dropped support for it. Depending on the version of Media Center you're using (and how old it is), you might try switching to the Chromium engine option and see if it works. Nonetheless MC28 adds Microsoft Edge as the default browser engine, which should hopefully solve your issue. You'll have to try it once it's released.
It might solve the issue above and it is safer than all other browsing engines.
I disagree, Chromium (which Edge is now based on) in general is more secure than Firefox, due to the sandboxing features Chromium has that Firefox doesn't yet support. I've had discussions with multiple security researchers in the field over the years, many of which also came to the same conclusion. While there has been improvements, the estimate is Firefox is maybe still several years behind Chromium in this regard. There's a website keeping track of Firefox's progress in regards to its overall security and sandboxing that's being updated constantly by a security researcher here:
https://madaidans-insecurities.github.io/firefox-chromium.htmlIn my opinion adding Firefox as a browser engine option to Media Center would be a step backwards, considering Microsoft Edge will do what both Internet Explorer and Chromium has in Media Center in the past, in addition to support DRM protected content including Netflix. There's no reason whatsoever to consider adding another browser engine option.