My mistake: this is not related to the removal of about:permissions, but to a new api for nsiPermission:

https://bugzilla.mozilla.org/show_bug.cgi?id=1173523

Unfortunately, as the thread above indicates the API changed in late 2015 but the documentation not yet
been updated:

https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Reference/Interface/nsIPermission
https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Reference/Interface/nsIPermissionManager