I have a similar problem, but on Lemmy.zip (Lemmy.world looks fine). However, I am running Fedora Kinoite (immutable), where Firefox is not installed as Flatpak.

I’ve been using Fedora Kinoite for a few months now. Before that, I also hopped between different distros quite a bit, including Ubuntu, Mint, Fedora Workstation, openSUSE. To be honest, I really appreciate the immutable nature of Kinoite and maybe that is why I am really happy with the experience. Even “normal” Fedora Workstation caused a lot of problems during my usage and with Kinoite, nothing like that so far. So I can really recommend it.
There’s only one really small problem I’ve noticed, which is a problem with the rendering of some fonts on web pages (e.g. lemmy.zip). These fonts are not rendered correctly, or are replaced with some default fonts, and they just look weird. There was nothing like this in openSUSE KDE, so I assume the problem is with Kinoite itself, as I have this problem on two machines. However, it is so rare (I only noticed it on two websites) that it can be ignored.