That was a DRM thing. Eventually Firefox added support for EME: https://www.pcworld.com/article/3183742/data-center-cloud/ne...
https://help.netflix.com/en/node/23742
You can see what resolution you're getting with the Ctrl+Alt+Shift+D shortcut to bring up Netflix's debugging information (and press again to dismiss it). You can also verify your resolution with the Test Patterns video.
The sad thing is 1080p video works just fine in Firefox. There's a Firefox add-on available which enables the 1080p stream:
https://addons.mozilla.org/en-US/firefox/addon/force-1080p-n...