https://support.mozilla.org/en-US/kb/hyperlinks-in-messages-not-working

   $ journalctl -xe     # on systemd systems, or take a look at the tail of /var/log/syslog .  And if you see a line like this:

   Jan 03 11:35:00 love2d kernel: 

   Jan  3 10:39:15 Love2d kernel: [78782.657577] 

  $ ps auxZ | grep -v '^unconfined'

  $ sudo aa-complain /usr/lib/thunderbird/thunderbird          # note: Insert the actual path here to your thunderbird if different
  Setting /usr/lib/thunderbird/thunderbird-bin to complain mode.
  Warning: profile thunderbird represents multiple programs

   $ sudo aa-complain     #with no parameters, you get a list of what AppArmor is watching and how.  For me there was no 'profile' for Thunderbird.

Here's a problem/solution. :-) The problem is the one described in my Question above. The solution is below. I'm hoping someone can append my solution to your page about links not working in Thunderbird emails, here: https://support.mozilla.org/en-US/kb/hyperlinks-in-messages-not-working _______________________________________________________________________ In Linux you might need to check if AppArmor is filtering the link click requests. To investigate if this might be your problem, just after you click on a link in Thunderbird run $ journalctl -xe # on systemd systems, or take a look at the tail of /var/log/syslog . And if you see a line like this: Jan 03 11:35:00 love2d kernel: audit: type=1400 audit(1578087300.218:3729): apparmor="DENIED" operation="exec" profile="thunderbird" name="/opt/firefox/firefox-bin" pid=8865 comm="gio-launch-desk" requested_mask="x" denied_mask="x" fsuid=1000 ouid=10 Jan 3 10:39:15 Love2d kernel: [78782.657577] audit: type=1400 audit(1578083955.569:2986): apparmor="DENIED" operation="exec" profile="thunderbird" name="/opt/firefox/firefox" pid=6897 comm="gio-launch-desk" requested_mask="x" denied_mask="x" fsuid=1000 ouid=1000 then you have a problem with AppArmor. Also you can run this: $ ps auxZ | grep -v '^unconfined' to report if in-fact Thunderbird is being 'confined' by AppArmor. If it is, then install the package apparmor-utils to get the aa-complain command, and run it like this: $ sudo aa-complain /usr/lib/thunderbird/thunderbird # note: Insert the actual path here to your thunderbird if different Setting /usr/lib/thunderbird/thunderbird-bin to complain mode. Warning: profile thunderbird represents multiple programs INITIAL SOLUTION: Convert AppArmor's 'enforced' mode to 'complain mode'. At this point my links started working again! For me this appears to because I'm running a more advanced version of firefox, and thus have it located in /opt rather than /user as it is from a package. If you run $ sudo aa-complain #with no parameters, you get a list of what AppArmor is watching and how. For me there was no 'profile' for Thunderbird. So far I have been unable to fix the AppArmor profile, but in time I'm come to understand it better. For now this gets Thunderbird working for me. Perhaps someone else can tell me how to adjust the apparmor files. I tried to edit /ec/apparmor.d/abstractions/ubuntu-browsers to update the paths for firefox, but this didn't fix my issue.