Rated 2 out of 5 stars
Doesn't seem to work at all. If I click on the gravatar, an address book entry opens up and it shows the correct person, but no picture (when in fact in addressbook it does), and it can't be edited anyway.
This add-on, like the header, uses the first contact it can find with the sender's e-mail address. I don't have any control over which contact is found first.
You can either attempt to remove duplicates (check out the Duplicate Contact Manager 2 add-on) or add photos to the duplicate contacts.
If you click on the gravatar to edit the person's picture you will probably have to re-select the message to see the new photo.
If it still doesn't seem to be working, please use the contact info (forum or e-mail) in the support section so I can find and fix the problem.