Hello,
For some reason I fail to understand, a non-devel package is
conflicting with a devel package :-/
According to dnf it's the only explicit conflict for the package:
$ dnf repoquery --conflicts firefox-55.0.2-2.fc26.x86_64
pkgconfig(nspr) >= 4.16
Maybe someone confused Conflicts with BuildConflicts in the spec?
Dridi