Hi I am not a Mac user, and the one I know in this forum is not on the version 10.15 Catalina release. You could try https://support.mozilla.org/en-US/products/thunderbird and search for Catalina to see which recent questions refer to the latest Mac OS. The results may put you in touch with others using Catalina. More generally - Be aware that Catalina specifically does not support any 32 bit app or component. If you use add-ons with Thunderbird, such as a calendar, those may be affected. Off-topic, but this may be helpful to you to know about other glitches related to Catalina: http://osxdaily.com/2019/10/09/troubleshooting-macos-catalina-problems/ It's a long article and people are still adding comments. Many of those reported are rare problems - but it is a useful resource to search for any specific glitches you encounter. As you are transferring to a laptop with Mac already installed, it may be smooth sailing for you. Agnes