I have my system configured1 to use JoyPixels for emoji, which I consider vastly more attractive than Noto Color Emoji. Sadly GitHub uses browser sniffing to detect Linux user-agents and replaces emoji with (badly aligned) images of Noto Color Emoji. They don't do this on macOS and Windows. In this post I explain how I worked around this.
The solution is simple: make GitHub think you're using a Mac or Windows PC.
There are various ways to change the User-Agent string of Firefox. The easiest
about:config but I didn't want it to be a global change — I
want sites to know that I'm using Linux in logs/privacy respecting analytics (I
block most trackers).
I ended up using the User-Agent Switcher and Manager browser add-on. I
configured its allow list to only include
github.com, and use the
User-Agent string for Firefox on macOS. The end result? JoyPixels, just like
P.S. If anyone from GitHub sees this. Please stop browser sniffing Linux visitors. Linux desktops and browsers have had working emoji support for years now.
I use the term, "configured", loosely here as all I really did was install the ttf-joypixels package.