It's not immediate obvious what the default MW firejail config does, nor is that /etc/firejail/mediawiki.local should be created (and potentially what should be in it, beyond saying to look at https://firejail.wordpress.com/features-3/man-firejail-profile/ for docs)
This applies both to MW and its documentation in the repo and also https://www.mediawiki.org/wiki/Manual:Shell_framework