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 :
/usr/lib/purebrowser/purebrowser mixr,
(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.
This worked for me as a temporary solution (sorry I am a Linux noob and I dont know if it might be the same solution you provided and just done in an other way?)
sudo apt install apparmor-utils
then after it was installed sucessfully, I typed:
sudo aa-logprof
it willl show the following:
Reading log entries from /var/log/syslog. Updating AppArmor profiles in /etc/apparmor.d. Profile: thunderbird Execute: /usr/lib/purebrowser/purebrowser Severity: unknown (I)nherit / (C)hild / (P)rofile / (N)amed / (U)nconfined / (X) ix On / (D)eny / Abo(r)t / (F)inish Should AppArmor sanitise the environment when switching profiles? Sanitising environment is more secure, but some applications depend on the presence of LD_PRELOAD or LD_LIBRARY_PATH. [(Y)es] / (N)o
I typed : Y
Then: S
Then: F
Now Thunderbird can open attachment again in the default application and I can also click on URLs and they will open in PureBrowser.
Thanks to Jens Erat at superuser.com
See:
https://superuser.com/questions/1276256/thunderbird-cannot-open-attachments-under-linux-any-more-after-updating
Greetings from a happy Librem 15 user in Switzerland