It's not surprising that this feels different than Thunderbird as it's a completely separate app with a separate background just being rebranded with Thunderbird identity. There's probably some effort to make them play nice together, but K-9's been around for ages (I think I've been using it for more than a decade).
And very negative for the people that selected K9 precisely because of the peculiar UX. 5.600 has been the last good version for me. I still sideload it on every new Android device I buy.
