Thunderbird randomly and irretrievalby deletes every message in a folder

I'm running Mac OS X 10.9.5 and Thunderbird 31.5.0. This has happened to me twice over the past couple of months. Once, I was trying to move all of my 10,000 messages from 2014 to my 2014 archive folder (which I do at the end of each year to clear out my inbox, sent, and trash folders) and Thunderbird randomly and irretrievably deleted EVERY message in the archive folder. All my messages for an entire year were gone. I tried repairing under preferences, tried manually deleting the .msf file, nothing worked. Then, two months later, it happened again, this time to my INBOX. I was working in Thunderbird in my Inbox and then the program froze for a moment and when it returned to life, all 40+ messages in my INBOX were gone. All my messages for an entire year were gone. I tried repairing under preferences, tried manually deleting the .msf file, nothing worked. This is obviously extremely disconcerting because it's wiping out very important work emails and I have no idea when it is going to happen. Please help.