works for me

defaults/pref/autoconfig.js

@@ -0,0 +1,7 @@
+// based on https://support.mozilla.org/en-US/questions/1378404#answer-1510431
+// apparently, this is a strange subset of JS
+// semicolons are required
+// basically, do not edit except for the filename below
+pref('general.config.filename', 'autoconfig.js');
+pref('general.config.obscure_value', 0);
+pref('general.config.sandbox_enabled', false);