Interesting, did Firefox revert their decision towards DNT? I remember being confronted with the behavior of it sending DNT headers no matter what, which must have been something around spring 2019.
Possibly, I'm not using the DNT header though, because of the encryption, I need to know about DNT before the analytics data is even sent. I use navigator.doNotTrack, and where it's set I only encrypt and send a minimal visit count event.
I tried to do the exact same thing and had to stop considering DNT as it would essentially collect Chrome only back then. Good thing they have reverted this I guess. DNT makes a lot of sense as a concept still.
IMO it's pretty much useless (and possibly even harmful) and a failed experiment. GoatCounter intentionally ignores it. Context: https://www.arp242.net/dnt.html