TBB 12: Customatization of settings via /etc/torbrowser.d/user.js does not work anymore

Qubes-Whonix
8

You can use policies.json:
http://forums.dds6qkxpwdeubwucdiaord2xgbbeyds25rbsgr73tbfpqpt4a6vjwsyd.onion/t/change-default-preferences-for-tor-browser-in-dispvm-using-policies-json/16722
Clearnet link:
Change default preferences for Tor Browser in DispVM using policies.json