1080p Rated 4 out of 5 stars

"The YouTube player uses separate streams for audio and video, which could be downloaded, but you would have to merge them using a software like ffmpeg." ialc

hey ialc, how can i do that? download 1080 separated files?

This review is for a previous version of the add-on (1.7.12.1-signed). 

No need for that, at least for now. Update to the latest version of the extension to get 1080p. Go to the Firefox add-ons section and "check for updates". The latest version is 1.7.12.