sorry for necroposting
I want to share a quite simple way to make a whonix friendly minimal changes to the tor browser config. It will allow you to update the browser to a fresh version with update-torbrowser (in whonix-15 template).
You can use >> for save the default effect of security-slider-highest.js file.
In whonix-dvm (template_for_dispvms):
user@host:~$ tail -n 2 /rw/config/rc.local
sed -i 's/#tb_security_slider_safest=true/tb_security_slider_safest=true/' /etc/torbrowser.d/30_default.conf
echo 'user_pref("browser.ctrlTab.recentlyUsedOrder", false);' > /usr/share/torbrowser/security-slider-highest.js