Rated 4 out of 5 stars

I don't like that it groups things erroneously without a way to manually override. And yes, I read the "about threading" wiki; I don't think "well, just tell people to write better emails/update their stuff" attitude by the developer is very useful in most business settings. Maybe in a software company or other tech environment where everyone is under 30. Not so much in the rest of the real world.
Otherwise, it's great. A few tweaks here and there, and it will be incredible. e.g., I wish I could customize the message toolbar, as I can without the extension. I like all my buttons aligned left.

This review is for a previous version of the add-on (2.0.8).