The package description for mozilla-firefox is:
Code:
# mozilla-firefox (Mozilla Firefox Web browser)
#
# This project is a redesign of the Mozilla browser component written
# using the XUL user interface language. Firefox empowers you to
# browse faster, more safely and more efficiently than with any other
# browser.
#
# Homepage: https://www.mozilla.org/firefox
#
We're now using Firefox Quantum, the whole point of which (as far as I know) is to remove XUL. In favor of what, I don't know. I just happened to notice that the package description is now out-of-date and probably in need of a refresh.