My gut suggests it’s a font issue, like librewolf is using a system font and firefox is using embedded or downloaded fonts.
backing that up with a search, I see there’s lots of people complaining of font rendering issues of various types in the librewolf subreddit: https://www.reddit.com/r/LibreWolf/search/?q=font
I don’t know what your fix is, but I hope this helps guide you.
Nah, that was worth watching.