Why don't you ask Incredimail? Do they support importing? If so, you can come back here and ask how or if Thunderbird can meet Incredimail's needs. This add-on: https://addons.mozilla.org/en-GB/thunderbird/addon/importexporttools/?src=ss offers several export formats. IMHO .eml is probably the most reliable. If your accounts use IMAP, you don't need to do any exporting or importing. Just add the account, using IMAP, to your new email client.