I switched from Mail.app to Thunderbird a couple years ago because I needed the ability to send GPG encrypted emails.

No surprise, Mail.app integrates much better with the rest of Apple software especially the Calendar. Also Mail.app does not prompt me to upgrade to a new version every week randomly disabling useful "incompatible-with-this-version" plug-ins like Thunderbird does these days.

Since the number of encrypted emails I exchange is very small these days, I decided to switch back to Mail.app.

In order to do that, I was prepared to write some conversion scripts after researching the emlx format, Mbox vs Maildir and some already written conversion script. I though stumble upon the following thread and realized I might just be able to import my Thunderbird mbox files directly.

I first localized the mbox files I cared about; that is the inbox and sent messages.

$ find ~/Library/Thunderbird/Profiles/ -name 'INBOX'
$ find ~/Library/Thunderbird/Profiles/ -name 'Sent'

It turned out I also needed all mbox files in the "Mail/Local Folders". That made 15 files to import total. Great! that is still manageable manually. Then I opened Mail.app, clicked File > Import > From Thunderbird, browsed to ~/Library/Thunderbird/Profiles/ and clicked OK. The mbox files I wanted to import where all there. I ended-up with an Import folder under "ON MY MAC" that I sorted into different archives folders. The sorting took a while but the whole process was so easy after all!

I also had to create HTML signatures for Mail.app for a few new accounts. It looks very much back channel but those steps worked. I found the key matching my signature and renamed the saved .webarchive as such.