That's achieved with the Lightning add-on, I believe.

I'd like to see Thunderbird focus on separating their mail handling engine from the UI aspects. Much like how the Chrome V8 engine can be used by a variety of projects, it would be nice if a separate open source project could take the Thunderbird engine, and experiment with their own UI.

It would also allow the adoption of newer replacements for IMAP. My hope would be that it could also lead to the replacement of proprietary messaging networks such as Slack, Discourse, etc., with something based on the newer open email format.

I think you mistaken Discourse with Discord. Discourse is open source forum engine, the second is Slack-like but faster, with rich permissions system.

Yes, you're right. I meant Discord. Thanks for the correction. Too late for me to edit though.