So when FF is totaly broken, I would:
delete ~/.mozilla drawer completely.
Then remove all versions of firefox including language-files - and reboot.
Install “Stacer” and use “System Cleaner” Tab to clean ALL “Application Caches”.
Maybe a conflict gets resolved so.
(less riscy than deleting contents of ~/.cache/mozilla)
?? => sudo pacman-mirrors --interactive --default && sudo pacman -Syyu
If no one has the solution - bumping won’t help - but it will cause the topic to be closed.
It has probably something to do with your Apple hardware.