This add-on does NOT insert emoji! It inserts tracking images, disguised as emoji! So every e-mail you insert them in, is going to be tracked! The maker of the add-on knows exactly who your correspond with, and even your emotions WRT those people!
It could have been done much safer, by using actual Unicode characters, aka *actual* emoji. Or at least, by using data: URLs to embed the images, and maybe the Unicode characters for clients that don’t display images.
Conclusion: DO NOT INSTALL!
This review is for a previous version of the add-on (1.5.4).
This add-on does not track emails. The src of the images inserted in HTML mode is set to the official CDN of EmojiOne, jsDelivr. No personal information is ever transmitted, i.e. neither email body nor email header information. The author of the add-on (me :) has no access to the CDN. I certainly do not know "exactly who you correspond with" nor do I want to know. This add-on is a non-commercial open source effort (free as in both beer+speech :) The source is on GitHub at https://github.com/mganss/EmojiAddIn
In HTML mode you can Shift-click to insert the Unicode character only. Even though by default in HTML mode images are inserted, the alt attribute of the inserted image is set to the Unicode character so if you copy the emoji the Unicode character will be on the clipboard. In plaintext mode the Unicode character is always only inserted.
There are two reasons I chose to use the EmojiOne CDN instead of data: URIs: First, it would have increased the size of the email by a few KB for each emoji used. Second, not all MUAs (email clients) support the data protocol (e.g. Outlook).
The reason I chose to use images (and not only characters) is also twofold: Not all operating systems support the latest Unicode standard (in fact none of the major ones AFAIK) meaning you'd get a placeholder if you clicked on one of the newer emojis. Also, the platform specific emojis look different from the EmojiOne ones which means you'd get a different looking emoji than the one you click on in the add-on.