Recently some websites started to use "system-ui" font-family which defaults to Cantarell on Fedora 33, and it looks too lean. After that website change, the fonts became less readable.
As I'm using KDE, there doesn't seem any way to adjust "system-ui" font for Firefox — it seem to support this function only with Gnome desktop. I had to change fontconfig configuration manually.
My proposal is to change "system-ui" to Arimo, for example.
<alias>
<family>system-ui</family>
<prefer>
<family>Arimo</family>
</prefer>
</alias>