I'm not sure if we should do this by default but a pref would be nice. The original use case for including contacts from disconnected accounts was for viewing logs once the conversation was open, I think (especially with infinite scroll in mind).

On the behalf of Florian: Closing bugs related to the Instantbird UI as WONTFIX, as the development of the standalone chat client Instantbird has stopped. Instantbird users are encouraged to migrate to Thunderbird. The user interface of instant messaging in Thunderbird will feel familiar, as the Thunderbird IM support started as a fork of Instantbird.