I am past the GRUB issue thanks to the pinned post shown here.
I edited the /etc/default/grub file to change the following line.
GRUB_CMDLINE_LINUX_DEFAULT="resume=UUID=d510aca6-3616-478f-bcba-d84439b35f82 loglevel=3 nowatchdog nvme_load=YES nvidia-drm.modeset=1"
After the edit, that line became:
GRUB_CMDLINE_LINUX_DEFAULT="resume=UUID=d510aca6-3616-478f-bcba-d84439b35f82 loglevel=3 nowatchdog nvme_load=YES pci=noaer nvidia-drm.modeset=1"
I then ran sudo grub-mkconfig -o /boot/grub/grub.cfg, but before that command could return, the bug I am trying to fix hit me.
I restarted the computer using the Magic SysRq Key as suggested on Sometimes, Endeavour OS stops opening new applications - #6 by Pudge, I re-ran sudo grub-mkconfig -o /boot/grub/grub.cfg (which was able to terminate), and I restarted my computer.
I can report the journal isn’t flooded anymore with those messages. I can see many messages I was not seeing before this change, and finally (I hope 
) understand what exactly the issue with Firefox is.