hello, those addons are neither developed by google or mozilla, but a third-party developer. you'd need to get in contact with them to inquire if and when they plan to make an extension available for firefox as well...

Where would you want to insert these emoji's?

Does BitMoji have a web-based interface where you can view emoji's in a browser? If yes, someone else could build a Firefox add-on to access them. On the other hand, if access to the icons requires some "secret sauce," then you're dependent on the company to build it.

I guess if all else fails, once you view the icon in, say, an email message created on a platform that supports BitMoji, you can right-click > Copy Image Location and use that address to insert the picture anywhere that pictures are allowed to be inserted by address.