Cannot open path of the current working directory: Permission denied bis

OK, so it doesn’t look like AppArmor is to blame here. Just for debugging, please try using the no_root_squash mount option, and see if that helps.

As another test, try doing

# let's first check the existing permissions:
ls -l /home/p00000012366/snap/firefox
chmod a+rwx /home/p00000012366/snap/firefox

and then starting firefox again.