Rated 4 out of 5 stars

Found the options buttons at the bottom of the information page. Description on Github link - " "triple style", which shows a single arrow (›) if a message is sent only to you, a double arrow (») if it is sent to a group and a triple arrow if it is sent to a mailing list."

Triple style works but seems a bit random: single arrow correctly shows up on a first email addressed to me only, double arrows appear on my reply to the sender of that email, (including an unsent draft), and (correctly) to an email to 1 with 2 ccs; triple arrows on an email received addressed to 2 emails and 2 ccs, and (correctly) to emails sent to a large list. Interestingly, only 1 spam message had 3 arrows: a spoof email sent from my own address)
It works with Header Tools Lite and picks up ccs (and my bccs - interested to see if it picks up SENDER bccs?) in its calculations , which is good.
I will continue with it, it is useful.

(When Gmail view is selected (the default), spam, email from me to myself, group and cc'd emails all show 1 angle bracket. (I have catchall mailboxes, as do most domain name owners, they work with the triple style.)
Same result when I sent it to a 1-off email address (not catchall).

Couldn't find how to "add email addresses as identities in thunderbird"? Does this mean have accounts settings set up for each email account (which I have, and the triple style works with that, I wouldnt want the Gmail style anyway)? I have catchalls set up for 2 accounts (each with aliases =different names@my domain name); and 1 email address for each of my other TB accounts.

This review is for a previous version of the add-on (0.7). 

Did you add all you email addresses as identities in Thunderbird?