Exactly. Since portable browsers are not registered in the registry, you need to import the ESET root certificate manually.
Even though the portable browser is not registered in the registry, ESET still can detect that the portable browser exists on the system. For example, under List of SSL/TLS filtered applications, all portable browsers I have are listed despite the root certificate not being installed. I think it would be much more user-friendly if ESET just imported the root certificate for all browsers.