Enable IMAP in GMail.com settings then add GMail and iCloud IMAP both to Thunderbird, then you can just drag and drop the messages from the GMail IMAP folders to the new iCloud IMAP folders in Thunderbird.
Enable IMAP in GMail.com settings then add GMail and iCloud IMAP both to Thunderbird, then you can just drag and drop the messages from the GMail IMAP folders to the new iCloud IMAP folders in Thunderbird.