Comment # 1 on bug 1229124 from Sofie F.

Created attachment 876857 [details]
Screenshot of Notion using bad font rendering in Firefox

Looking a bit into this, it seems related to the fact that Noto Color Emoji has
definitions for both digits and space, and the fact that Notion writes their
font list with the emoji font before the sans-serif fallback openSUSE
Tumbleweed falls back on for Liberation Sans by default.

While disabling the /etc/fonts/conf.d/49-family-default-emoji.conf can work as
a workaround, we are actually removing the alias between Apple Color Emoji to
Noto Color Emoji, which is what seems to be causing this issue. Replacing Apple
Color Emoji with Noto Color Emoji on Notion stylesheets gives the same result.

For solving this, I'm not sure what the right approach would be, but its likely
going to involve Noto Color Emoji package.