Would it be possible to create a script to save the desktop and screen position for each firefox window on close and restore them to the recorded desktops and positions on open? I haven't used i3 much (or arch at all), but I'm fairly certain I did something similar using Xubuntu some years back.

I'm almost sure this is an excerpt straight from https://youtu.be/7Nj9ZjwOdFQ

Keep in mind that you can downgrade firefox back after the update with pacman -U /var/lib/something, then just refresh/keep using the open windows with no issue.