In the update that I did today Icecat is no longer Deterministic.
I compiled Icecat since there doesn't seem to be any substitutes. After 
the second time it wrote an error that the two builds were not the same.
My system is this:

Generation 13	Jun 25 2020 16:46:39	(current)
  guix d1a26be
    repository URL: <https://git.savannah.gnu.org/git/guix.git>
    branch: master
    commit: d1a26bedcc63362609604ec36c7a5ebee825d140

Marinus