Review for Auto Avatars by Hudojnyk
Rated 4 out of 5 stars
Works well as described. I'm not sure whether it will fetch and try display Gravatars for emails not already in your address book (if not please consider adding that too).
Also consider adding BIMI (Brand Indicators for Message Identification) support. I have to use this together with another addon called Auto Profile Picture to get both brand and Gravatar images, but it would be amazing if this could be just 1 extension.
To create your own collections, you must have a Mozilla Add-ons account.