Continuing the discussion from [qubes-users] Tor Browser "Safest" as default setting:
On my system, the simpler solution within this Whonix Forum post https://forums.whonix.org/t/add-tor-browser-first-startup-popup-to-ask-whether-security-slider-should-be-set-to-safest/7591/8 works:
“… create a file /etc/torbrowser.d/50_user.conf with the follow contents …”:
tb_security_slider_safest=true
I have this in whonix-15-ws and it survives updates.