MacMini: slow updated Thunderbird to version 68.4.2:

on Thunderbird invitation, updated my Thunderbird to v.68.4.2 on my late 2014 macMini with the latest OS; performance of the updated Thunderbird is very poor, including slow display (18 sec delay to show the Thunderbird mail from behind Safari files) and the spinner going for similar time for every click on the  mail.  Asked the Mozilla community for help but so far not much useful info came back.  One person suggested that the Catalina was a potential problem.

Would appreciate help.