So, I _just_ fixed this issue.
On my reasonably exotic setup⁰, I just realized emojis did not display properly. They worked on my terminal (Alacritty) and seemed to work on Qt-based things, but did not work on the ONE place I would expect them to work: Firefox.
After following some essential troubleshooting steps, venting about them a lil' bit, and getting nudged towards Pango, I finally found a Gentoo bug report that seemed to show a very similar issue.
The solution was not related to anything I just wrote.
The solution was to disable the "Infinality" fontconfig configuration that Gentoo still provides as an option for some reason, since they are _ancient_. I would've expected them to either keep it updated, or remove it entirely.
And just like that, emoji started to appear 🎉🎉.
Conclusion: Don't use old software, especially if you know they're old.
I finally set up a compose key binding so I didn't have to copy-paste special characters to use them. That seemed to work on everywhere except Alacritty. I was just about to ask _here_ why this might be, when I decided to check on Alacritty's issue tracker again.
The solution was to use `en_US.UTF-8`¹ instead of `C.UTF-8` as the system locale.
0: (Currently) Gentoo with ALL use flags disabled by default (USE="-*")
1: Or probably any other locale that isn't `C`
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