If you're on Linux and apparmor is running AND you, like me, want to also be running the latest version of Firefox, then the trouble is that the Thunderbird apparmor profile is mis-configured for this setup.

To fix this:

1) Add this line to /etc/apparmor.d/usr.bin.thunderbird:

 /opt/firefox/firefox-bin Pux,    # <-- note:  
                  # adjust the path and name to your firefox

2) Important: Now restart apparmor to re-load this profile using: $ sudo /etc/init.d/apparmor restart

Note: this means you trust your version of Firefox, as it bypasses the default security handshake between Thunderbird and the older version of Firefox it wants to call. But given that I keep the latest version of Firefox installed it's most likely that it has more security holes patched then the previous version of Firefox which doesn't have those holes patched. It's a judgement call here, and there IMHO is no really right security answer.

And YES this DID solve the problem, at least for me.

But NO, I won't be clicking the "Did this reply solve your problem? [Solved the Problem]" button here, which locks this thread to others who might have other solutions to later add.

This prior question/answer was unfortunately "locked", meaning no other or better! answers could be posted for it. (IMHO this locking of threads is a very bad practice leading to many unhelpful and outdated hits when searching for an answer to a question.)

In my opinion the question was asked and answered in 2017. Even thinking the person concerned was still interested in "another" answer after six years is a stretch and it is for this reason topics are automatically locked after 6 months in inaction. Is the information dated, obviously. Bu8t other than sending an email to the original poster, you will achieve nothing, except a complaint from those being spammed by your response years after the fact.

The internet is full of dated information. It is something I love about it. I can research old stuff if that is what I want to do. Microsoft and a new generation of web cleaners want only the most recent information. Something that is rather frustrating if the information you want pertains to windows 98, something I still need to look up for.

If you want to do something useful with this information, update the hyperlinks do not work support article to include the information. It is years since it was updated and then it was me tinkering around the edges. https://support.mozilla.org/en-US/kb/hyperlinks-in-messages-not-working

Note that the Linux specific information should be enclosed in a showfor and that you have to explain the how to do it in detail. Assume the reader has no idea how to do more than open the application.