Rated 5 out of 5 stars

I also really like this add-on. Much better looking than the new icons introduced in TB 78. As someone else mentioned however, I would really like to see you remove the envelope icon from the subject field. There only needs be an indicator if you have replied to the message. We can already tell if a message has been read or not by whether or not it is bold. An icon is unnecessary. The indicator would be easier to see if it was just an arrow rather than an envelope with the M or whatever that is. If you don't want to change this, could you offer an option to disable this icon except for the replies?

Edit: Thanks for the code for the .css file. I really appreciate it!

This review is for a previous version of the add-on (3.0b5). 

The answer is no, sorry, if you want to disable this icon (reset it to default) add this to userChrome.css:

/* ..... restore default subject column icons ..... */

treechildren::-moz-tree-image(subjectCol) {
list-style-image: none !important;

treechildren::-moz-tree-image(subjectCol, forwarded) {
list-style-image: url("chrome://messenger/skin/icons/forward.svg") !important;

treechildren::-moz-tree-image(subjectCol, replied) {
list-style-image: url("chrome://messenger/skin/icons/reply.svg") !important;

treechildren::-moz-tree-image(subjectCol, replied, forwarded) {
list-style-image: url("chrome://messenger/skin/icons/reply-forward.svg") !important;

treechildren::-moz-tree-image(subjectCol, new) {
list-style-image: url("chrome://messenger/skin/icons/newmail.svg") !important;

treechildren::-moz-tree-image(subjectCol, imapdeleted) {
list-style-image: url("chrome://messenger/skin/icons/cancel.svg") !important;