I'm trying to figure out how to remove the recently added emoji reactions to posts. I don't care about them, and each one moves the next post down a little, wasting screen space.

I can do it in ublock origin with the filter:
quokk.au##.message_reaction

So I tried to modify the CSS from this post's example .fe-warning { display: none; } into .message_reaction { display: none; }. But that doesn't work, and I'm a little lost where to go from here (I don't really know what I'm doing in the first place). Thanks for any help!

You were really close, you need to hide the outer container that the .message_reaction is in. Here you go:

.message_reactions {  
    display: none;  
}  

Nice! Thank you very much.