I just updated my packages today and saw this.
apt-listchanges: News
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thunderbird (1:52.5.0-1) unstable; urgency=high
The profile for AppArmor within Thunderbird is now disabled by default.
Since the activated Recommends for the apparmor package in recent
kernel-image packages it turns out there are to many things that simply not
working like before if Thunderbird is using the current AppArmor profile.
Please read README.apparmor for getting more information about the
re-enabling of the AppArmor profile.
People who wants to help to improve the AppAprmor profile are appreciated.
Take also a look at #882048 for the reason behind that decision.
https://bugs.debian.org/882048
Can somebody explain what this means security-wise?