The [http://kb.mozillazine.org/Importing_from_Incredimail recommended method] uses [https://github.com/smowton/incredimail-converter-reynardware this program] to convert iml to eml. [https://turgs.com/blog/incredimail-to-thunderbird/ Here] is another app that converts to mbox format. Both eml and mbox can be imported to TB, either manually or with [https://addons.thunderbird.net/en-US/thunderbird/addon/importexporttools-ng/ ImportExportTools NG]. Drag eml files from an Explorer folder to a subfolder of Local Folders in TB. Or, copy mbox files into the Mail/Local Folders location of the profile folder, while TB is closed.