Enable DRM Playback on Firefox
It shouldn't come as any surprise that Disney Plus, like most other streaming services, employs DRM to prevent unautorized use of their content. As a result, you're going to enable DRM playback on Firefox, if you haven't already.
This is a companion discussion topic for the original entry at https://linuxconfig.org/how-to-watch-disney-plus-on-linux-with-firefox