IMAP is awesome, but has some rough edges around things like IDLE being only able to monitor one folder per connection. This means users may get an inconsistent experience depending on the number of folders they have, with emails arriving in some showing up immediately, while others have to wait until the regular "check for new messages" check.
JMAP (for example) removes these problems, and for mobile applications, provide a better experience.
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Cleaner, simpler. Maybe faster. Yes, please.
This looks like a good option however there is little mainstream server-side support for this protocol which means there is not much for TBird to work with. Chicken and egg ... And although Cyrus is one of my favourite mail servers, it's not really mainstream. Let's see if there is movement on the server side of the next couple of years.
In fact, there is also a 7 year old bugzilla bug for this one: https://bugzilla.mozilla.org/show_bug.cgi?id=1322991