1. Disable /dev/tcp in bash. Compiled time: --enable-net-redirections, --disable-net-redirections
ksh and zsh seem to have it.

Tests:
a)
cat </dev/tcp/time.nist.gov/13

b)
exec 3<>/dev/tcp/www.google.com/80
echo -e "GET / HTTP/1.1\r\nhost: http://www.google.com\r\nConnection: close\r\n\r\n" >&3
cat <&3

c) A list of attacks
http://www.lanmaster53.com/2011/05/7-linux-shells-using-built-in-tools/

2. SELinux integration

Firefox selinux disabled (RedHat): http://danwalsh.livejournal.com/72697.html
Firefox selinux enabled (Gentoo hardened): http://blog.siphos.be/2015/08/why-we-do-confine-firefox/
"desktops are notoriously difficult to use a mandatory access control system on"

3. abstract unix socket bridge, example for ibus:

before the sandbox is started
socat UNIX-LISTEN:/tmp/mysoc,fork ABSTRACT-CONNECT:/tmp/dbus-awBoQTCc &
in sandbox
socat ABSTRACT-LISTEN:/tmp/dbus-awBoQTCc,fork UNIX-CONNECT:/tmp/mysock

5. add support for --ip, --iprange, --mac and --mtu for --interface option

6. --shutdown does not clear sandboxes started with --join

7. profile for okular

8. profile for dillo
Also, in dillo open a directory (file:///etc), when the browser window is closed the sandbox still remains active.
This is probably a dillo problem.

9. --force sandbox in a overlayfs sandbox

$ sudo firejail --overlay
# su netblue
$ xterm &
$ firejail --force --private
Parent pid 77, child pid 78
Warning: failed to unmount /sys

Warning: cannot mount a new user namespace, going forward without it...
Child process initialized

Try to join the forced sandbox in xterm window:
$ firejail --join=77
Switching to pid 78, the first child process inside the sandbox
Warning: seccomp file not found
Warning: seccomp disabled, it requires a Linux kernel version 3.5 or newer.
$ ls ~ <----------------- all files are available, the directory is not empty!

10. Posibly capabilities broken for --join

$ firejail --name=test
...
$ firejail --debug --join=test
Switching to pid 18591, the first child process inside the sandbox
User namespace detected: /proc/18591/uid_map, 1000, 1000
Set caps filter 0
Set protocol filter: unix,inet,inet6
Read seccomp filter, size 792 bytes

However, in the join sandbox we have:
$ cat /proc/self/status | grep Cap
CapInh:	0000000000000000
CapPrm:	0000000000000000
CapEff:	0000000000000000
CapBnd:	0000003fffffffff
CapAmb:	0000000000000000

11. check seccomp on Docker: https://docs.docker.com/engine/security/seccomp/
Seccomp lists:
https://github.com/torvalds/linux/blob/1e75a9f34a5ed5902707fb74b468356c55142b71/arch/x86/entry/syscalls/syscall_64.tbl
https://github.com/torvalds/linux/blob/1e75a9f34a5ed5902707fb74b468356c55142b71/arch/x86/entry/syscalls/syscall_32.tbl

12. check for --chroot why .config/pulse dir is not created

13. print error line number for profile files in  profile_check_line()