> How 'recent' are the packages?
It's the only place I know of where you can currently get GCC 7.1. [0]
> Is it possible to run Firefox (not Iceweasel)?
If you spun up your own package, and didn't distribute, probably. The IceCat package is 746 lines of Scheme at the moment, so porting it might not be complete hell. [1]
That being said, the current IceCat is based on Firefox 52, so that's a fairly close version-parity, even if you miss out on the nonfree aspects of FF.
> libvirt
Guix currently has libvirt 3.2.0, whereas the very latest is 3.3.0. A fairly close parity, and I'd expect an update to the Guix package soon-ish.
[0] https://www.gnu.org/software/guix/packages/g.html
[1] https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages...