First, I don't know anything about Macs. BUT, if Macs are like Linux and Windows, there should be a folder named Thunderbird that contains a profiles.ini file and a folder named profiles. If so, the backup process is - exit Thunderbird (Important) - using the OS's file manager, copy the thunderbird folder to external media. The advantage of copying the thunderbird folder and not just the profile is that if you ever need to reinstall, it's a somewhat simple process to - install thunderbird and exit - copy the backed up thunderbird profile to replace the default established during setup.

AND, Macs may be totally different - but I had good intentions. :)