Rated 5 out of 5 stars
The intuitive features work as expected without requiring a lot of trial-and-error to figure out how to use it.
In addition, it stays up to date and compatible with Firefox, Thunderbird and, dare I say, Chrome. With an eye for unexpected or unwanted behavior, I have seen neither over several years. People complaining about unwanted installations should RTFM and not click stuff just because it says "click me".
This add-on is a trusted standard.