There's an old discussion on this that was closed back in April 2017.
It looks like 1P still doesn't handle numeric IPv6 addresses for auto-fill. This is not a big deal yet obviously -- but eventually it will be.
I have several local servers that I access via numeric IP -- and while I can certainly use IPv4 addresses now, I am trying to constantly move in the IPv6 direction to be future ready.
I don't have a local DNS setup, nor do I want to go to very machine and add this to a hosts file.
I hope there's an open defect report for this -- hopefully scheduled to be fixed "at some point."
1Password Version: OS X 7.2.4
Extension Version: Firefox 188.8.131.52
OS Version: OS X 10.14.3
Sync Type: N/A