install userChrome.css using prefs.js instead of schell scripts
Firefox's preference files are Javascript, and as such 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...)
Inspired by the following article: https://mike.kaply.com/2016/05/24/default-profile-directory-doesnt-work-in-firefox-46/