Looks like some poor subpixel rendering. You might want to try looking for some settings to tune it.
Given that you're using a version of Arch Linux, Firefox and focusing on one particular site, It's probably only just you.
Re:Faking Twitter unfurling to phish you
by nightfeather on Monday November 08, 2021 @08:13AM
(#29148902)
happens to have a same setup as you, I don't have such problem.
https://i.imgur.com/ZP9wC85.png