Previous comment is wrong, there is every kind on emoji, flags, dogs and whatever and it's pretty fast.
I would love to have keyboard shortcut so I don't have to break my workflow while typing.

No pleading eye/puppy eye emoji, no emoji of country flags, no category of emojis.. have potential but lacks much

This is probably the best of the emoji addons for Firefox. It has built in categories for logical organization. There are 3 rows of categories at the top and then there's a drop-down menu next to the search bar. I kinda wish the 3 rows of categories could be switched out with some of the categories from the drop-down menu.

It's better than Emoji keyboard because there's no empty white space at the top or the space inefficient design.
It's better than Unicode Emoji because the emoji are distinct and categorized.
It's better than Emoji Cheatsheet/Emoji helper because there are no fake emoji.

It is simply the best.