The default apparmor configuration shipped with PureOS does not allow links in Thunderbird to be opened in PureBrowser.
To fix, add this line to /etc/apparmor.d/local/usr.bin.thunderbird :
(note; the trailing comma is required)
Then “sudo apparmor_parser -r /etc/apparmor.d/usr.bin.thunderbird”
PureOS should enable this by default in /etc/apparmor.d/usr.bin.thunderbird, as it is expected behavior and both Thunderbird and PureOS are pre-installed.