thunderbird: fix launching preferred browser

Vincas Dargisrequested to merge
Talkless/apparmor-profiles:thunderbird-xfce into master

In Debian 10 with XFCE desktop this denial is produced [0]:

May 12 17:43:07 tbirdtest audit[2307]: AVC apparmor="DENIED"
operation="open" profile="thunderbird"
name="/home/gnoutchd/.config/xfce4/helpers.rc" pid=2307
comm="exo-helper-1" requested_mask="r" denied_mask="r" fsuid=1000
ouid=1000

In the result, Thunderbird launches only "sensible-browser", not the one user has chosen as preferred one.

Add file rule to fix launch of preferred browser.

Closes Debian 960465.

[0] https://bugs.debian.org/960465

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