Just checked and can see them on my desktop computer and phone but on the tablet only see black boxes?
All three use firefox browser.
As I suspected, it appears to be the OS. Likewise, I am sure if you went back to Win 7 or earlier some emoji's would not display, despite the browser. I can't link from my tablet easily, but go to Wikipedia and look at their emoji page and links and see emoji's designed for various OS.
As a rank amateur I relate the emoji thing to the same category as Win 'Special Characters'. As the OS systems advance in sophistication, the special characters expand. Each special character has a display code number. Newer special characters won't display on an older system. The same is true with emoji's; if you look in 'Preview' when posting on FR, you should see a #code in the text field in place of the emoji you entered being displayed in 'Preview'. To my ether brain it is that code# that some OS are unable to decode. .... or perhaps the Iditerod dogs just haven't arrived yet... you know.