firefox: fix file-dialogs and icons with gtk3

There was the usual crashing and a few icons missing.
@lethalman: I think it's best to start putting $XDG_ICON_DIRS into suffix
instead of prefix (as here), so user-installed icons take precedence.

/cc #7743.

pkgs/applications/networking/browsers/firefox/default.nix

@@ -95,6 +95,11 @@ stdenv.mkDerivation rec {
 
       # Remove SDK cruft. FIXME: move to a separate output?
       rm -rf $out/share/idl $out/include $out/lib/firefox-devel-*
+    '' + lib.optionalString enableGTK3
+    ''
+      wrapProgram "$out/bin/firefox" \
+        --prefix XDG_DATA_DIRS : "$GSETTINGS_SCHEMAS_PATH:" \
+        --suffix XDG_DATA_DIRS : "$XDG_ICON_DIRS"
     '';
 
   meta = {