mate (GTK3) gives the same problem here, also with Thunderbird email (which does shows icons but not (e.g.) inbox contents. Clicking on message list shows actual message OK)
Installed chromium, which works fine. Tried several themes with no joy (mate & Mozilla).
Also installed Openbox DM, both FF & TB ok with that.
Rod’s Books gives in-depth information on UEFI
Rod has developed rEFInd (which is in Arch’s repository) - Arch-wiki entry rEFInd states:
rEFInd is a UEFI boot manager capable of launching EFISTUB kernels. It is a fork of the no-longer-maintained rEFIt and fixes many issues with respect to non-Mac UEFI booting. It is designed to be platform-neutral and to simplify booting multiple OSes.