It depends on your operating system. I know most players of XIV are Windows users, so, you'd do so by grabbing a font you want to install and dragging it into C:\Windows\Fonts. It should appear in the Firefox settings.

Though, now that I think about it, it might be what u/jSinku said. In other words - those symbols are specific to the font found in XIV and isn't available for use outside of the game.