How are fonts handled?

[NWoodsman]

I'm on NixOS running Mango and Firefox looks awful because it's reading from some system font but i can't understand if Mango/Wayland/NixOS is setting the font. Can someone comment on if this is a Mango/Wayland issue and how to set the system font?

Edit: I am referring to the appearance of the application itself, not the web content. The tabs, menus, etc have the awful font.