Most people don't realize this but you can only watch Netflix in 4K in a browser if you're using Microsoft Edge on Windows or Safari in macOS 11. In fact, you can only get 1080p if you're running Chrome in Chrome OS, or IE, or Safari. All other browsers, including Chrome and FF on macOS, Linux, and Windows, are stuck at 720p.
Their blog post[0] from 2017 sounded hopeful for higher resolution video on Linux - they wrote "We... look forward to high-resolution video being available on more platforms soon" almost immediately after announcing FF on Linux was supported - but in hindsight it's clear that what they really meant was what you point out - support for things like Apple TV, Xbox, and TV Smart Apps.
[0]: https://netflixtechblog.com/update-on-html5-video-for-netfli...