Feature summary
Option to set cookies using hostOnly property.
Quote https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/cookies/Cookie
A boolean, true if the cookie is a host-only cookie (i.e. the request's host must exactly match the domain of the cookie), or false otherwise.
For example, hostOnly protects example.com cookies from being read by bar.example.com according to my understanding.
Since MediaWiki supports httpOnly, sameSite already, this seems a good fit for completeness of cookie security features.
Use case(s) / Benefits
Better security, privacy and defense in depth as in it might prevent yet unknown attack vectors.