Lemmy uses the system default for monospace font.
Try changing the monospace alias in /etc/fonts/local.conf:
https://wiki.archlinux.org/title/Font_configuration/Examples#Default_fonts
That's for system-wide effect.
For just firefox, go to Settings > General > Fonts > Advanced and
change the default Monospace font to a monospace font you like.
Source Code Pro and DejaVu Sans Mono are both very good.