The change requested comes from https://github.com/canonical/firefox-snap/pull/8
- etc-firefox-policies: + etc-firefox: interface: system-files - read: [/etc/firefox/policies] + read: [/etc/firefox]
The /etc/firefox/policies access was initialed added to get policies working, https://bugzilla.mozilla.org/show_bug.cgi?id=1785278 is another usecase for accessing files in /etc/firefox which triggered the change.
It seems logical for firefox to have access to /etc/firefox so we are requesting to give access to the directory instead of adding another exception for a subdirectory.
The change landed in nightly now which should reach the edge channel first.