Rated 5 out of 5 stars
A very valuable add-on.
If, like me, you're having problems importing windows-live-mail emails into thunderbird with this add-on, try this:
Get eml2mbx, a command-line freeware (that means, you write a single command in the windows/dos "command prompt" or in the windows/run)
Once you have your emails in the eml format, convert them to mbx format, in one batch, including all the folders and subfolders, and after that the add-on will import them into thunderbird (without the nested folder structure, however, but all folders will be there). To import into Thunderbird, create a new folder, click on it, and call the import/export addon from there (it won't work if you haven't clicked into a folder).