1. Find an unused user group for hidepid exception:

$ id
uid=1000(netblue) gid=100(users) groups=100(users),10(wheel),90(network),
92(audio),93(optical),95(storage),98(power)

From /etc/group I pick up a group I am not part of:

$ cat /etc/group
[...]
xmms2:x:618:
rtkit:x:133:
vboxsf:x:109:
git:x:617:
[...]

I'll use group 618 (xmms2)

2. Set hidepid and allow xmms2 users to bypass hidepid

$ sudo mount -o remount,rw,hidepid=2,gid=618 /proc
$ cat /proc/mounts | grep proc
proc /proc proc rw,nosuid,nodev,noexec,relatime,gid=618,hidepid=2 0 0

3. Test "firejail --list", "firejail --top", "firejail --tree", "firejail --netstats"