Popstate.dat is most often simply deleted when it becomes corrupted. This of course leads to a re-download of all messages in the Inbox. While it might be possible to avoid this by modifying the file, it's probably easier to open the account in webmail and move messages to another folder, that won't be downloaded with a fresh dat file (POP only downloads the Inbox).

I am locking this topic. Please avoid creating multiple postings on the same topic. Please continue in https://support.mozilla.org/en-US/questions/1503819