Really good:
i tried a lot of different 'overrated' extensions: no one BUT THIS SIMPLE ONE was able to 'Accept-Language' with the proper Locale

Doesnt really do anything with request headers

Exactly what i was looking for... same as the chrome extension !

Für norwegisches TV https://tv.nrk.no/direkte/nrk1 "X-Forwarded-For 160.68.205.231" funktioniert ModHeader nicht mehr. In Firefox wie auch in Chrome nicht.

Doesn't work with private window

Best Header editor

Finally an extension on Quantum that effectively blocks X-Frame Options allowing me to use Adobe Target in Firefox again - simple to use, does exactly what it promises.