Something that emerged from the recent "Gnome default web browser" 
thread on here: it looks to me like, in general, if

- an application is among Gnome's startup applications; and
- the guix package manager upgrades the package for that application

then the Gnome autostart system continues to point to the old version of 
the application.

This is possible because, unlike other distros, Guix's package upgrades 
leave the old package present on the system and install the new package 
in a different location in the filesystem.

Should this be considered a Guix bug?