If you have been trying to import your emails from MS Eudora or Outlook with the current Thunderbird 38.x branch you likely have run into problems. According to Bug 1176748, this functionality was disabled as it broke the Thunderbird 38 branch. There is however, a simple fix and that is to temporarily install Thunderbird 31.8.0 (or Thunderbird 17.8.0), import then update to the current Thunderbird 38.x branch. One catch though, this only works for Outlook. Apparently the import functionality for Eudora has not worked in a long time and since it has been almost 10-years since Microsoft ended support for Eudora, Mozilla is not going to restore this functionality.
At this time, it is not know when the import issues with Outlook are going to be corrected. It is doubtful that this will be working in the Thunderbird 45.x branch due out in Mid/Late March 2016. But again, it is fairly simple to install the older version of Thunderbird over the current version (this will not affect your profile) and then once you are done importing (since this only needs to be done once), manually download and install the current verison of Thunderbird via the getthunderbird.com site.