Rated 5 out of 5 stars

This is really nice. What's missing is some indicator whether an email uses HTML. That indicator could be a small symbol that appears in the header bar or in the status bar (e.g. together with the "Allow HTML" symbol), every time an email has an HTML part. Often it is not possible to tell whether an email uses HTML or not. I believe that's what also "Apedemak" meant in his comment below (from Jan 2010). (BTW, an old email client, "Foxmail", used such an indicator)
Edit May 14th:
Maybe, simply by not displaying this add-on's changing icon in the status bar, whenever a message is in plain text, would be enough to indicate that a message has no html part. I keep finding myself clicking on the html option in the status bar while some messages just don't have html.

This review is for a previous version of the add-on (3.3).