The NetBSD wiki on XFCE4 was last edited 6 years ago.
A more recent 3rd party guide is here:
https://slice2.com/2019/09/21/howto-...on-netbsd-8-1/
On my 9.2 install w/ 2021Q3 binaries, I ensured I had the daemons (ie dbus) running and mine was fairly stable.
Arch has a listing of things that cause problems in linux
https://wiki.archlinux.org/title/Xfce#Troubleshooting
One issue in the Arch Linux guide is the login manager or lack of one. I used xdm because the ~/.xsession-errors log was helpful. Consider setting up xdm if you are not using it.
I ran firefox-esr and did not experience major issues.