Or one could just use firejail, which comes with a number of pre made profiles for common applications.

Xlibre (the only current actively developed implementation of a X11 server) has a new extension - XNamespace to address some challenges as well.

https://github.com/X11Libre/xserver/blob/master/doc/Xnamespa...

This is a great article.

I have little experience with lxc but I guess waypipe could be an option too.

For an article written late last year I hoped for a little more awareness of how massive a security hole granting full, unfiltered access to the X11 server is. Granted, any sandboxing is better than none, but firefox is one of the few apps that already sandboxes itself really well, and with a blog title like that it might be good to touch upon things like nested X servers such as Xephyr.