Netflix screenshot quality

Hello,

I want to record a movie on netflix in 1080P on mozilla, only when I finish capturing the screen the resolution is in 1280x720 while I checked 1080p.
I do not understand because on this same series, sometimes it writes in the properties of the episode that it is in 1920x1080 and sometimes 1280x720, I do not know what makes the change:
I put in full screen and I start.

I have 16GB of ram.
I have intel (R) Core (TM i7-8565U CPU @ 1.80GHz 1.99 GHz
I have 367 GB free

How to do ?
Thank you