I don't know the answer to the question, but I'll clear up this 
sidequest:

Csepp 写道：
>> substituting 
>> /gnu/store/n6w5qizdwx2kpas061dcn5gdgvaqlwr1-firefox-91.13.0esr.source.tar.xz...

[…]

> Firefox is not even officially packaged, so that's the only 
> thing I can
> think of.

IceCat is built from Firefox ESR sources, using the ‘makeicecat’ 
script and other modifications.  If there are no substitutes for 
the modified source, this processing can happen locally, which 
would download the original.

See 
<https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/gnuzilla.scm#n483>.

That said, icecat 91.13 is 3 months old.  Somebody's Guix needs an 
update.

Kind regards,

T G-R