Location of system installed firefox addons?

I am trying to package tridactyl and while I can get tridactyl.vim@cmcaine.co.uk.xpi to show up in /run/current-system/sw/lib/firefox/browser/extensions/, I can’t get firefox to pick it from there.

Here’s my tridactyl/default.nix

    channel? "stable", # Only option for now
stdenv.mkDerivation rec{
    pname = "tridactyl-firefox";
    version = "1.16.3";

    src = fetchTarball {
        url    =  "https://archive.archlinux.org/packages/f/firefox-tridactyl/firefox-tridactyl-${version}-1-any.pkg.tar.xz";
        sha256 = "0zfrlmycpzank2b9apdcn6bxa3j5jbhv2w7p2xx9dmhj6zc07mc8";

    installPhase = ''
        mkdir $out;
        mv lib $out


I would think firefox would recognize that location, but I’m not sure what I may be doing wrong…