Import OLM to Thunderbird is not possible directly. But here’s an excellent alternative approach that you wished you had known before.
Import Outlook 2011 to Apple Mail with “OLM Extractor Pro”, you will exact copy of original file without any data loss. Try it today for your evaluation.
Here’s a handy tool to import Olm to Mac Mail. Brought by USL Software, this will let you convert your data with ease and efficiency.