Firefox internally only supports bool, int, and string types for
preferences, but often stores objects, arrays and floats as strings.
This change makes it nicer to specify those type of preferences in
Nix, and it also makes it possible to merge objects & arrays across
multiple modules.
This makes the
programs.firefox.package
option take a pre-wrapped Firefox package as value if state version is
set to "19.09" or later. This should make the Firefox module work with
a wider range of Firefox packages.