This post is going up not because it is particularly interesting, but because Google failed to be particularly helpful in solving this problem for me.
If you want to do things the other way around (Evolution to Thunderbird) then you won’t be short of resources.
Evolution has an import tool, nestling away under File > Import
Go on. Try that. It looks promising.
The thing opens up and assures you that it will guide you through the process of importing external files into Evolution. You follow its prompts and check the box to import data and settings from older problems. You discover it has less use than expected.
Apparently it searches for setting from Netscape, which is what Thunderbird is based on. What gives?
At this point I went ‘Gah!’ and turned to my good friend Google. Google! Why have you forsaken me?
What I should have done was turn to Evolution’s internal help files. They don’t look immediately useful either, but on closer inspection you will find a section on Migration from Outlook to Evolution.
Now this is sort of silly, but to migrate from Outlook to Evolution, you first need to import your data into Mozilla Mail (which is (kind of) Thunderbird)).
The reason is some drivel about the necessary library only being available on windows.
Come on Novell, you know that’s stupid. If you expect people to migrate from MS products to OSS, then you need to make it simpler than that. But I digress.
Your Thunderbird mbox files are hiding out here (well, mine are):
~/.mozilla-thunderbird/(random letters)/Mail/Local Folders/
They are the files without extensions, with names like Inbox, Sent, Drafts and so on.
If you’ve gone and made nested subfolders in your inbox (or any other folder for that matter), you’re in for a frustrating time because it is necessary to import each mbox file individually (unless you can write a script to do it, but if you could then you wouldn’t be reading this poorly explained effort).
If you have subfolders in Inbox, change directory to Inbox.sbd
One again the mbox files will have the same name as your folders. You’ll have to import each one, one at a time.
Welcome to the suck!
Here is how you import an mbox file now that you’ve found them:
Back to that largely useless import tool I mentioned earlier. It has an ‘import single file’ option. Guess what?
That’s right. Then browse to the folder and open it. If you’re lucky, the stupid importer will recognise that the file is indeed in mbox format. It might not. I found that sometimes it did, and other times it didn’t. Keep trying until it works, or alternatively, smack your computer with a sledge-hammer. Your call.
Assuming it works, click ‘Forward’ and then select the destination folder. Fortunately you are given the option to create a new folder, so that may bring a little relief in your world of pain.
Enjoy, you masochist you!
The Waffle Group 
I would ask “why are you migrating from Thunderbird to Evolution?”, but I suspect that might cause me to be hit with a hammer.
It does give the best results, I’m told.