Some monospaced text does not properly render capital E (it appears as a blank space). This is independent of any extension or setting (persists after deleting ~/.mozilla), and does not appear in the Firefox flatpak.
I am using an up-to-date install of Manjaro GNOME with nvidia graphics on Xorg.
ABCDEF. On my screen this is is indistinguishable from ABCD F.
Yep, ttf-dejavu is installed. The problem goes away if I force Firefox to use a custom font for all monospaced text instead of allowing sites to use their own fonts. Changing the default font does not help.
My only guess now is that something must be wrong with my fontconfig settings, though I haven’t altered them by hand on this install, as far as I can recall. I tested a fresh Manjaro GNOME live image, and Firefox was working properly there.