Rated 3 out of 5 stars
There's a very important feature you haven't rewritten: preferred unanswered first. Without this, the thread is not indented correctly.This review is for a previous version of the add-on (0.3.3).
I do have it.
The extension does support this. You can sort the messages in the review dialog according to their headers, with the unanswered being first (or last). Then 'Keep First' does the trick.