Generally, see Do not start GUI Applications with Root Rights - because that creates file permission issues. Also don’t use sudo to manipulate files in /home/user folder - same reason.
Command
sudo chown --recursive user:user /home/user
fixes permissions issues in user home folder.
firefox-esr
(Debian decision to call it that way.)
but not advised as per Tor Browser Essentials
KVM? See this Whonix / Kicksecure KVM 15.0.1.3.4 Released
a means alpha as in “testing”.
That text is already saying “In most cases you are best off choosing the lowest version number among them.”
I don’t know. It’s a Debian / XFCE question. Best resolved as per Self Support First Policy for Whonix