Rated 5 out of 5 stars

Mimika's suggestion does the trick. But when I use Thunderbird as part of PortableApps the Import tab in the Preferences window does not work. Eg. the button next to the pref.js radio button has the label "Select" but it is dead and you cannot browse your directories nor you can write anything into the text field either.
Any suggestions?

This review is for a previous version of the add-on (0.8.35).