How can I run firefox using buildFHSUserEnv?

I want to write a custom package which runs firefox under a buildFHSUserEnv (I want to be able to keep it up-to-date faster, and hopefully I’ll be able to keep it somewhat sandboxed).

Right now, my firefox.nix looks like:

{ pkgs,glib, gtk2, gtk3, gnome3, gsettings_desktop_schemas, wrapGAppsHook, ...  }:
with pkgs;

let
    firefox = stdenv.mkDerivation rec {
    name = "firefox";
    version = "68.0.1";

    src = fetchurl {
      url = "https://download-installer.cdn.mozilla.net/pub/firefox/releases/${version}/linux-x86_64/en-US/firefox-${version}.tar.bz2";
       sha256 = "09j94c18c7wk13rc7w6ycbrykf9081rpx7i7qvja1is7n82rf3wb";
     };
     installPhase = ''
       cp -r . $out
    '';
 };
in
  buildFHSUserEnv rec {
     name = "firefox-exe";

     targetPkgs = pkgs: with pkgs; with xorg; [
       gtk3 librsvg makeWrapper
       wrapGAppsHook
       gsettings_desktop_schemas glib
       gnome3.defaultIconTheme
       gnome3.adwaita-icon-theme
       gtk3-x11
       libX11
      gtk3
      libXt
      dbus-glib
      alsaLib
      atk
      cairo
      curl
      cups
      dbus-glib
      dbus
      fontconfig
      freetype
 #      gnome3.gconf
      gdk_pixbuf
      glib
      glibc
      gtk2
      kerberos
      libXScrnSaver
      libXcomposite
      libXcursor
      libxcb
      libXdamage
      libXext
      libXfixes
      libXi
      libXinerama
      libXrender
      libcanberra-gtk2
      libnotify
      libGLU_combined
      nspr
      nss
      pango
      libheimdal
      libpulseaudio
      systemd
      ffmpeg
     ];

     runScript = ''
       ${firefox}/firefox
     '';
   }

It works fine. The only thing is, that when I try calling “save”, firefox crashes and I get:

(firefox:27035): GLib-GIO-ERROR **: 20:11:44.310: No GSettings schemas are installed on the system
ExceptionHandler::GenerateDump cloned child 30292
ExceptionHandler::SendContinueSignalToChild sent continue signal to child
ExceptionHandler::WaitForContinueSignal waiting for continue signal...
[2]    27030 trace trap  firefox-exe
Exiting due to channel error.
Exiting due to channel error.
Exiting due to channel error.
Exiting due to channel error.

How should I fix this?