If you are having problems in importing your emails from Outlook to Thunderbird (Free Alternative to Outlook), then here is a good solution, which may let you do that easily.

Import Emails From Outlook To Thunderbird

If you are using Outlook for a longer time period, and now you are trying to shift to Thunderbird, but unable to import the emails to Thunderbird easily, through its wizard, because of large size or any other issue, then let me tell you a good, simple and easy solution.
All you need is (the old) Outlook Express, which is usually installed by default in the Windows XP. Follow the steps below to get the solution:

• As direct Import from Outlook to Thunderbird is not successful in some cases, first you need to import your Outlook emails to the old Outlook Express
• Now open Thunderbird, and try to import your emails from Outlook Express to Thunderbird, by going on Tools -> Import and selecting Outlook Express, instead of Outlook

After waiting a few minutes or seconds, you will get your emails imported to the Thunderbird.

Hope the solution works for you to successfully import your emails from Outlook to Thunderbird.