Unfortunately, Thunderbird does not offer an export option yet. Because of that, importing your old mails to Evolution is not made that easy. But, luckily enough, Thunderbird uses a standard mail box format: mbox. mbox format is supported out of the box by Evolution through its import feature.
We know that we can easily import mails from thunderbird, the only thing left, is to find out where those emails files are. The location can be found through the account properties page. In Thunderbird go to Edit--->Account Settings... and select the account you want to import the mails from, then select the sub-entry Server Settings and look for the Local Directory text field. It should look like:
If you ‘ls' this directory, you will find a collection of files and directory. You will most likely have to do an ls –a to see the hidden directory. If you look at Inbox for instance. We have:
* Inbox.sbd directory
Inbox.sdb is a directory containing all subdirectories and their messages that you might have create under Inbox folder in thunderbird.
Inbox.msf is a file containing a summary of your emails for better performance.
Finally, Inbox file contains your mails in a mbox format. This is the file you want to import in Evolution.
Let's import our Thunderbird Inbox to Evolution. In Evolution click on File--->Import. This will open up a wizard window. Click Forward, choose Import a single file, click forward again. In the new window, open up the file browser, and make your way to /home/user/.mozilla-thunderbird/gfdse2df.default/Mail/mail.server.com .
Note: You may have to right click in the file browser window to enable the "Show Hidden Files".
Select your Inbox file and that's it. Evolution is going to import your thunderbird files.
If you created a lot of sub directories, you will have to go through every files (without the .msf extension) and sub-folders under Inbox.sbd.