Firefox's use of Encrypted Media Extensions violates the GNU Free System Distribution Guidelines as it allows for playing of DRM-protected content which can steer people to non-free software (there may be additional other reasons but that's one I definitely know of).

But it's perfectly available in NonGuix; I use Firefox regularly on Guix.

As for Librewolf, I'm not sure that's accurate? I see it available in the default Guix repo.s, latest version: https://packages.guix.gnu.org/packages/librewolf/.

It was added about 2 years ago. Admittedly, Guix is a smaller distro so some packages aren't packaged immediately just as a consequence of a lack of human resources; but it also continues to improve over time, quite nicely.