[mildred593]

What would be the benefit to using JMAP in thunderbird?

JMAP main benefit is that it allows a mail client to to things efficiently in JMAP that are extremely difficult to do in IMAP or take a long time to do. Merely adding JMAP support in Thunderbird will not add any benefit.

The benefit will come if Thunderbird is refactored internally to make use of the nice properties of JMAP, but then there will be a problem with IMAP compatibility which must be kept.

JMAP is nice for new mail clients that can move away from a folder-centric paradigm, but if you already managed to use IMAP then it's less useful.