How to import Outlook folders to Thunderbird? Get the download professional Outlook PST to MBOX Converter software. It is one of the best applications to instantly bulk import Outlook folders to Thunderbird open source application. During the migration process, the tool keeps and maintains all the folder structure without any changes and data loss. It has a very simple GUI, everyone can easily handle and manage the software without having any technical knowledge. There is no file size limitation, one can export a large size of PST files into Thunderbird without any data loss. It was compatible with all the latest and below editions MS Outlook, Windows OS and Mac OS.