The MSF files denoted by the extension .msf, is the Mail Summary File of Thunderbird emails. It only contains the headers and the summary of emails. Thunderbird stores the mails in two files- one that holds the actual mail (MBOX File) and the other that holds the index of that particular mail. This index file is referred as the MSF file. The main function of this file is to make sure that the other applications import mail from the MBOX file without any extension.