Vault "timeout" using Firefox

Norton Security 360 with Identity Safe: Using Internet Explorer, my Vault stays open until I manually close it or reboot the computer. 

Using Firefox, I thought it was annoying enough that my Vault only stayed open until I exited Firefox, but it seems to be worse than that. On a number of occasions, I have discovered that my Vault got closed after a day or so, even though Firefox was never closed. Is there some timeout default, or is this a bug?