Update: I don’t really know how but it seems that the apparmor profile was the culprit since even uninstalling the AUR firefox version and installing the official one wasn’t helping.
So I deleted all what I did in this solution for the "Failed to start Load AppArmor profiles” error.
At this point I would again have had apparmor error if using the ubuntu version, then I installed the firefox-appmenu-bin from the AUR and it’s currently working as expected, even though I had to make GTK_USE_PORTAL=1 works again.
In conclusion, if someone follow this nice tutorial and decides to install the firefox-ubuntu-bin, be aware of the chance to see the apparmor error and that if you solve it (maybe that solution wasn’t good but the error disappeared) it might mess with widevinecdm.