I just switched from Ubuntu 23.10 to Debian 12 (using X11) if that helps. I didn’t have this problem in Ubuntu.
Are you using Firefox within a flatpak perchance ?
There seems to be a bug to it relating to use of bitmap fonts, you can fix the issue by disabling them via a config file in firefox’s fonts: https://bugzilla.mozilla.org/show_bug.cgi?id=1621915
From your other responses, this is a system issue not a problem with the website.
Lemmy.world’s code has this font list for sans-serif: system-ui,-apple-system,“Segoe UI”,Roboto,“Helvetica Neue”,“Noto Sans”,“Liberation Sans”,Arial,sans-serif,“Apple Color Emoji”,“Segoe UI Emoji”,“Segoe UI Symbol”,“Noto Color Emoji”
I’d use the dev tools to check which font is being rendered. I’m on Windows so I get Segoe UI, which I find entirely acceptable.
You could start with using dark reader
