This comes from the inclusion of abstractions/ubuntu-browsers.d/firefox that in turn includes /etc/apparmor.d/abstractions/ubuntu-browsers.d/user-files:
# Allow read to all files user has DAC access to and write access to all
# files owned by the user in $HOME.
@{HOME}/ r,
@{HOME}/** r,
owner @{HOME}/** w,
This comes from the inclusion of abstractions/ ubuntu- browsers. d/firefox that in turn includes /etc/apparmor. d/abstractions/ ubuntu- browsers. d/user- files:
# Allow read to all files user has DAC access to and write access to all
# files owned by the user in $HOME.
@{HOME}/ r,
@{HOME}/** r,
owner @{HOME}/** w,