Hey, at least it told you. When i was fiddling to get disney plus working on my own setup it just refused to play until i changed the reported OS to windows. Nothing else was changed. I really have no idea why they would go out of their way to block specific operating systems.
You did? I also tried that, but the best resolution I got was 540p... As far as my research went that was because they require a higher widevine level not available on linux for HD
That's more to do with the version of WideVine DRM your browser has - the DRM in Firefox is neutered so the chrome will always give the best experience. Why? Because the WideVine DRM is owned by Google.
I have regularly watched Disney plus and Max on my Linux systems. But not all Linux systems are equal. Watching it on something like Debian would be pretty hard. Debian generally doesn't support much in the way of DRM as it goes against most of their philosophy. You can get a browser installed with support for the widevine DRM they require. But it's a lot of work on a system like that. However under Arch or Endeavor OS it was relatively simple.