Hello, this happened after upgrading the kernel from 4.19.12 to 4.20.0. Full output is below and I have no idea where even to start looking (same thing happens with apparmor disabled):

firejail --x11=xephyr --apparmor --name=firefox --net=br10 --profile=/etc/firejail/firefox.profile openbox --startup /usr/lib64/firefox/firefox

*** Starting xephyr server: "Xephyr" "-ac" "-br" "-noreset" "-screen" "800x600" "-title" "firejail x11 sandbox" "-resizeable" ":363" ***

The XKEYBOARD keymap compiler (xkbcomp) reports:

Warning: Unsupported high keycode 372 for name ignored
X11 cannot support keycodes above 255.
This warning only shows for the first high keycode.
Errors from xkbcomp are not fatal to the X server

*** Attaching to Xephyr display 363 ***

Reading profile /etc/firejail/firefox.profile
Reading profile /etc/firejail/firefox.local
Reading profile /etc/firejail/firefox-common.profile
Reading profile /etc/firejail/disable-common.inc
Reading profile /etc/firejail/disable-devel.inc
Reading profile /etc/firejail/disable-interpreters.inc
Reading profile /etc/firejail/disable-programs.inc
Reading profile /etc/firejail/whitelist-common.inc
Reading profile /etc/firejail/whitelist-var-common.inc
Error send: arp.c:182 arp_check: Invalid argument