At a loss trying to figure out any mechanism whereby Firefox would
blacklist the router IP range, which appears to be what's happening.
Is it possible to go back and use *any* other IP range you set the
router to before ? I.E. is this blacklist being kept somewhere ?

If the list is being stored then the hosts file would be a likely
source so have you investigated that route ? Look in /etc/hosts.deny
too as well as /etc/hosts itself.