Need to convert your Maildir structure into a single mbox format file, perhaps so you can easily dump it into Thunderbird? Use the handy command formail that comes with procmail:
for i in new/* cur/*;do formail <“$i” >> ../mbox;done
Run this command from from the top of your Maildir (usually ~/Maildir), and you’ll find a file called mbox in the parent directory once it is finished.
Want to import that new mbox file into your Thunderbird profile? Open up Thunderbird, right-click the Local Folders item, choose Properties, and look for the location of the Local Folders directory in your filesystem. Now, copy that mbox file to that directory, then close and reopen Thunderbird.