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I don't think they have a way for users to fund individual features, but you can help feature development in general by donating: https://give.thunderbird.net

Since the most recent comments indicate that they have it on the roadmap, I think that would also help the specific feature you care about.

Dumb question, but could you find an open source contributor who knows the codebase and commission them to implement it?

I've been looking for something like this for numerous projects. If I can't write the code myself, I'd love to pay someone who can, but I don't know where to look.

I completely agree. This is what I check every time there’s news about Thunderbird before I consider switching.

What are you using currently? I am limping along with Thunderbird myself but really miss three-pane view.

That already exists in thunderbird, what do you mean?