Sure,

  guix package --search='web browser'

should return a bunch of interesting answers.

IceCat is fork of Firefox.
Other than that, you'll find a bunch of webkit-based Epiphany, Eolie, Vimb, etc.

More browsers are coming.

Chromium is available in a separate channel:
https://gitlab.com/mbakke/guix-chromium.

We don't have Thunderbird nor any fork (yet).
There is claws-mail and maybe you'll find a few others with a search query.

-- 
Pierre Neidhardt
https://ambrevar.xyz/