Group by discussion without tree in Thunderbird

Question

Is it possible to group messages by discussion (with keep sorting by date) without tree? In other words: limit discussion tree by 2 (1 level - top message, 2 level - all other messages in selected sorting order).

Nearest solution:

1. turn on Subject sorting
2. turn on Group by sorting method

It looks as wanted, but this solution has next disadvantages:

1. Any changing in subject creates new group
2. Discussion sorted in unwanted way

The Conversation extension is not solution too:

1. Time to time it marks unread messages as readed without me o_O
2. Too slow

Any extensions and recommendations is welcomed.