If you are looking for an Emoji add-on you really only have two choices. This one "Emoji Keyboard." OR "Emoji Cheatsheet"

But there are problems with both of them. The "Emoji Keyboard" has ALL updated emoji's, including different skin pigmentation (or race pigmentation.) BUT regardless, while using it it doesn't work properly with some website (i.e. facebook, etc) . . it will almost always mess up anything you've written when adding the emoji's, and forget trying to duplicating the same emoji (adding the same emoji numerous times.) . . than adding a different one will mess up everything you might have just written. Moving on. . .

Emoji Cheatsheet works well with almost any website. BUT lacks respect for other races with not having different skin pigmentation (or race pigmentation.) It's 2017 and it's time to get with the program and stop living in the 19 Century!
Other cons with this add-on are that some of the emoji's listed don't even work and again, it is not updated with all updated emoji's available.

My advise is simply stop using both of them! By doing so, maybe the developer will realize that they should update the add-on. All add-on's that are not kept up-to-date on Firefox should be removed from it's website. It is Hypocritical for Firefox to update itself but not expect developers to keep there's add-on's updated also!