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.
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.
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.