Navigate to your PROFILE folder in Thunderbird on the "old" account. I can't remember offhand the file path, but it's not the one associated with Programs. Copy that profile, which will be a random string of letters and numbers.
Paste that profile into ~/Library/Thunderbird/Profiles
Create the "Profiles" folder if you don't already have it.
All settings, themes, extensions, filters, flags, and mail, etc should now be in the "new" account on the new computer.
This also works for FireFox, including bookmarks, userchrome, themes, etc. Worked flawlessly when I set up TB and Fx on the Macbook, and wanting everything "the same" as what I had customised on the desktop Mac, which I had originally copied over from a WinXPpro version of the install, which I had run for over 5 years.
Note of course, that you will have to install TB for Mac for this to work. Once you've got access to everything on the same computer, "getting used to" Mail.app isn't as scary, IMO.
I tend to use TB and Mail.app pretty much interchangeably these days.