Preferences set with `pref()` can be changed by the user, but they're
then reset on each Firefox startup.
As users may want a different UA and keep it persistent, we should set
the default value with `defaultPref()` instead. This requires moving the
preference to the autoconfig file though, as `defaultPref()` isn't
recognized in the main config file.
Firefox can run an autoconfig Javascript on startup, which can be used to
install and update userChrome.css, both when creating a new profile and
using a pre-existing one.
This removes the need for a wrapper script and related complications
(changes to $PATH, different processing for new and pre-existing
profiles...)
Co-Authored-By: Oliver Smith <ollieparanoid@postmarketos.org>
