Firefox supports as language settings //Norwegian// with the language code `no` and //Nynorsk// with the language code `nn`. When you set as preferred language `no` as first language and `nn` as second language you get the HTTP header
According to https://www.iana.org/assignments/language-subtag-registry `no` (//Norwegian//) is a macro language with the languages `nb` (//Norwegian Bokmål//) and `nn` (//Norwegian Nynorsk//).
MediaWiki normally substitutes the language code `no` by `nb`.
ULS does not support the fallback from `no` to `nb` for the Accept-Language HTTP header and ignores `nb`:
$ curl -L -H 'Accept-Language: no,nn;q=0.5' 'http://localhost/mediawiki/' 2> /dev/null | head -2
<html class="client-nojs" lang="nn" dir="ltr">
Expected result is `lang="nb"` or `lang="no"`.
This bug is related to T183295.