Try "actual" content videos. You can press ctrl+alt+shift+D to open a debug overlay that shows the playback resolution.
The actual DRM limitations also vary by content (and region) - with some titles I get 720p on Linux while some other titles are limited to SD, while I get 1080p on Windows Edge on those same titles.
They may not have the same DRM restrictions as actual content? It's not that Firefox is not technically capable of rendering the videos, it's that it doesn't give the DRM the control that Netflix and rightholders want to require in exchange.
The actual DRM limitations also vary by content (and region) - with some titles I get 720p on Linux while some other titles are limited to SD, while I get 1080p on Windows Edge on those same titles.