Thunderbird 91 has threaded views in individual accounts. It is a little hard to enable the view though: In the header bar directly above the list of messages, there is a little "thread step" icon on the left by default, that must be clicked to enable thread view. This is next to the star icon, the paperclip, Subject, etc. Unlike the other header items, clicking the "thread" icon does not sort by threads, instead it toggles the thread view on and off.